Like every other group of hobbyists, brewers are susceptible to upgraditis. If you start picking around enough forums or meeting other homebrewers (drinking other brewers' beers is a wonderful thing), you'll see enough shiny things to spark a case of it. You probably brew on a budget, which upgrades eat into. Bang-for-your-buck is key. These are my suggestions, garnered from experience and browsing forums for the past year. The first three improvements are echoed on forums, daily. It's like calling tech support and being asked, "Is it plugged in? Have you tried resetting it?" They are absolutely the first things you should do and will result in better beer.
Fermentation Temperature Control
When you read the information on a packet of yeast, it generally tells you an acceptable range of temperatures for fermenting at. If you're a bit thick like me, you think this means to set the thermostat in the fermentation room to that temperature and ferment away. Problem is, as the yeast chew up the sugars in your beer, their metabolism generates waste heat. If uncontrolled, the heat is generally retained in the beer, causing its temperature to rise. As soon as the second day, the temperature of the beer can rise to 10 degrees F over the level of the room, throwing it right out of the range you wanted it in.
Temperature matter because yeast produce different ester profiles based on temperature(and the gravity of the beer and yeast population density, but those will be addressed momentarily). Generally, given a constant wort gravity and yeast population density, more esters will be produced at higher temperatures and fewer esters at lower temperatures.
By manipulating the temperature of the beer, you can manipulate the ester profile. Different esters are generated at different temperatures. Generally, lower temperature fermentations tend to just produce background esters, which provide a good backbone; higher temperature fermentations produce more pronounced esters. Some yeasts produce generally the same level of esters, but with a different balance at each temperature point; German wheat yeasts generally produce more clove esters at lower temperatures, with the balance moving towards primarily banana esters at the higher end of the range.
Equipment: Two primary methods of temperature control exist, swamp coolers and refrigerators. Swamp coolers are significantly cheaper, but require a little work to maintain; swamp coolers are not generally suitable for making lagers unless you have an extremely cold place to put them. Refrigerators are significantly more expensive (if you don't have a spare fridge) and are pretty simple to work once setup; fridges are capable of proper lager fermentation temperatures.
Swamp cooler: You will need a container which is watertight and similar in height to, but larger in diameter than your fermenter, a thermometer and a few two liter bottles. Prior to brew day, fill your two liter bottles about 2/3 full and cap lightly, then freeze. Once you get the beer into the fermenter, move the fermenter into the larger container. Make sure you have a large amount of water available at the temperature you want to ferment at. Fill the container with the water to just below the liquid level in the fermenter. Once the fermentation is out of its lag phase - when the yeast stop replicating and start fermenting; say 12 hours, tops - plop one of the two liter ice cubes in the container. You will need to watch the temperature of the container and determine how often you need to add the ice and how much ice you need to add.
Refrigerator: In order to use a fridge, you will need a temperature controller. Johnson manufactures the most common single-stage temperature controllers, making both a digital model and an analog model. Ranco manufactures two-stage temperature controllers. Single-stage controllers are only able to control heating or cooling; dual-stage controllers can control both. More enterprising and electronics-savvy brewers have homebrewed their own controllers. Setup is easy: Empty out the fridge (and freezer, if it has one), turn it to full cold, plug it into the controller, plug the controller into the wall and place the temperature probe on the fermenter; if you want, you can use a thermowell to submerge the probe into the wort for better accuracy. Adjust the settings on the controller to the desired temperature. To heat it, put your heat source near the fermenter, place the probe inside it and punch in the settings.
I currently use a refrigerator and a Johnson digital single-stage temperature controller and am experimenting with different configurations of the pair. I believe a lower setting on the fridge and tighter differential on the controller will result in much lower temperature swings and even out the fermentation temperature. No results in on this yet, but I will be brewing more batches this year and next year to test out the theory. As with all things homebrew: Many possible methods exist.
Important note: After six or seven days, you should raise the temperature of the wort a few degrees - cease adding ice or crank up the setpoint of the controller a few degrees. At this point, most ester-generating activity will have slowed immensely, so allowing it to rise will not cause great harm. Doing this allows the beer to go through a diacetyl rest. Diacetyl is a byproduct of fermentation at lowered temperatures and will add a buttery character to your beer; allowing the fermentation temperature to rise near the end will allow the yeast to process diacetyl into something else, thereby removing the buttery flavor.
Pedantic note/stupid digression: The range of temperatures prescribed by a manufacturer is not the range of temperatures 'favorable' to the yeast; it is more accurately described as favorable to beer production. It's actually very rare that you ferment at the temperature that the yeast prefer, because the ester profile generated at that temperature is usually awful. I can think of two yeast strains that operate at those temperatures in a manner that brewers find useful: Sake yeast (used by few, if any, homebrewers, but Epic Ales in Seattle, WA uses it) and some Saison yeasts. True Saison yeasts are utter monsters - one forum poster jokingly said, 'WY3711 will eat small children and animals, if they get too close,' and was not far from the truth. Both yeasts operate at insanely high temperatures, sometimes going up to 100 degrees F, while still producing very good beer. Anyhow, I digress.
Control Yeast Pitching Rates
The next step you can take in making better beer is to control your yeast colony's size relative to the amount of wort being fermented and the gravity of the wort. Ideally, controlling the amount of yeast present allows you to dictate the background ester profile; for more extreme cases, the combination of temperature control and yeast pitching rate control will allow you to dictate foreground ester profile (or lack thereof). Either way, if you aim to brew to a style or clean up your beer's flavor profile, yeast pitching rates are important.
Pitching rates are measured in terms of population density or yeast cells per milliliter. On
Wyeast Labs' website (http://www.wyeastlab.com/hb_pitchrates.cfm), they list ranges from 6 million cells per milliliter to 24 million cells per milliliter. Again, according to Wyeast, higher specific gravity calls for higher yeast density and lagers generally call for higher densities than ales. By keeping these in mind and referencing the table on Wyeast's site, you can pitch the proper amount of yeast.
Brewers use one of four methods to get proper colony sizes. At the most cheap/awesome end of it, you can ask a microbrewery if they have yeast slurry they are willing to give away. If you're frugal and really like a yeast, you can pull yeast slurry from a previous batch (assuming it's relatively fresh). Next up in dollars is using a starter, effectively a small beer, to generate yeast slurry. If you don't want to put much effort into it, you can simply use more yeast packets.
Your best friend in all this is going to be some form of calculator for determining how much slurry to pitch/yeast packets to buy/starter wort to make. I suggest Jamil Zainasheff's calculator (http://www.mrmalty.com/calc/calc.html). It's excellent, very easy to use, and is free. Note that Jamil disagrees with Wyeast on pitching rates - big surprise, people disagreeing about brewing methods - and will give you different numbers from the Wyeast website. Anecdotally, I will say his calculator has never done wrong by me.
Getting slurry from microbreweries: It's as simple as asking and there's no love lost if you can't get any. Generally, you will have to provide them with a vessel that can be sanitized, they will fill it and seal it, then you pick it up. Some breweries are protective of their yeast strains and won't share because of that. One of the breweries near me offered up some cool Whitelabs yeast (I think it was an Abbey ale yeast) last year, announcing it via Facebook. Also, don't ask if they have a specific yeast strain, just ask if they have any they would like to give up - they are doing you a favor, so no kvetching about their yeast.
Pulling your own slurry: If you're brewing back-to-back-to-back, you can really save money by repitching yeast from slurry. Using this method, your yeast selection will be limited to what you recently used. The basic gist of all the 'rules' surrounding this is simple: If the yeast were stressed in the last ferment, you shouldn't repitch them. You know the yeast were stressed if they were in high specific gravity wort, high or low temperatures, or if they threw off-flavors or bad aromas. If your yeast is good and your sanitation is good (if it's not, fix that before anything else), you should be in good shape to repitch from the yeast cake at the bottom of your last brew.
Ideally, you will be brewing and racking on the same day. Once the beer is out of the fermenter (with the yeast cake left behind), harvest the cake into a sanitized container. Pull up Jamil's calculator and switch to the, "Repitching from Slurry," tab, enter the specs on the slurry and determine how much you will be adding to the new beer. Brew, pitch the slurry, and relax.
Making starters: Only for liquid yeast users or people who have very small yeast samples. Making starters is pretty simple, but you can overcomplicate things easily. For the basic gist of it, you will need a growler or a 1 gallon fermenter, a cap for it, some aluminum foil, dry malt extract and yeast. Again, Jamil's calculator is where it's at. There is a dropdown box listing a few different kinds of starters. It's fairly unlikely you have a stirplate, so you will have to have somewhat sizable starters; pick, "Intermittent Shaking," if you can afford to shake the starter up every hour or so. I usually do a volume somewhere between what I get from his Simple Starter versus his Intermittent Shaking settings.
To make the actual starter, find out how much starter you will need from Jamil's calculator, then weigh out enough malt extract to get that volume of starter to a specific gravity of about 1.030-1.040. It's not terribly important to be precise, just not too low or too high. Boil the starter wort for 15 minutes, cool it and pitch your yeast into your sanitized growler or fermenter. Sanitize the cap to it and a 6"x3" strip of aluminum foil, as well, then seal and shake the snot out of that sucker. Uncap it, pour in your yeast. Fold the aluminum foil in half (to get a 3"x3" piece) and very lightly cover the opening of the fermenter. Place it somewhere warm (~70degF) and let it go for a day. If you dedicate yourself to sealing (with a re-sanitized cap) the growler every hour or so, you can get away with that smaller wort volume.
Again, there are two ways to proceed. The first school of thought (which I subscribe to) is that the starter will probably taste like crap, since its job is to grow the yeast, not make good beer. If you believe that, cap it and put it in the fridge when you begin your brewday. When time comes to transfer from the kettle to the fermenter, reserve about 2 cups of wort. Pull the growler out of the fridge and pour off all the clear liquid, stopping as soon as it runs cloudy. Pour in half of the reserved wort, shake up and pour into the fermenter. Repeat with the other half of the reserved wort.
The other school of thought is lazy - when it comes time to inoculate the wort, just shake/swirl the starter and dump the whole thing in.
Use more yeast: This one is the simplest. Pull up Jamil's calculator, enter the parameters you will be using, then look at the number of vials or packs needed to get enough yeast. If you're using dry yeast, click on the, "Dry Yeast," tab to see how many grams of dry yeast you will need. Dead simple.
It turns out you can over-sanitize. It also turns out that the method used to sanitize your water - if you live in a water district - can be harmful to your beer as well. Chlorine and chloramine are common sanitizers, with the latter becoming even more common. Chlorine boils out or will come out if you let water sit out overnight. Chloramine is much more difficult to deal with. Campden tablets offer an extremely quick and cheap solution to both. I paid about 3USD for a bag of 30-ish Campden tablets at my homebrew store. Cents per batch for a bit of extra peace of mind. One tablet added to up to 20 gallons of water will nigh-instantaneously clear it of chlorine and chloramine. I add a tablet to my hot liquor tank and use it as my source of water for everything headed into the kettle.
"You can work hard or you can work smart." Brewing without a wort chiller is working hard; brewing with one is working smart. Lager-brewers will definitely need a wort chiller, as moving through the dimethyl sulfide-generating temperature range quickly is key to not having dimethyl sulfide in your end product. Wort chillers come in many different shapes and forms, ranging from the bulky-but-wallet-friendlier forms to the small-and-wallet-thinning forms. There are two major 'types' of wort chillers: Immersion chillers and counterflow chillers.
Immersion chillers: These are generally just a giant piece of copper tubing, x-many feet long, which you run cold water through. Typically fed with a garden hose. They're extremely easy to assemble on your own - just go down to the hardware store, find a 25 foot length of 5/8" ID copper tubing and a Sharkbite that will fit on it with a garden hose adaptor and voila. Bend the chiller into a coil that will fit in your kettle, put the Sharkbite on one end (above the level of the wort!), and aim the other end toward where a drain will be. If you can't just aim the exit of the chiller into a sink or drain of some sort, you can add another hose, but just grab some 5/8" high-temp tubing and clamp it on there, no need for a Sharkbite. Also, if you are chilling inside, you will need a garden hose adaptor for your faucet.
On brewday with 15 minutes left on the boil, attach the garden hose to the chiller then put the chiller into the boil. The Sharkbite will absorb a large amount of heat, so you will not be able to grip it to attach a garden hose later on. If necessary, unplug the garden hose from the faucet. Once the boil is finished, start running cold water through your chiller and stir the wort. Stirring greatly speeds up the process. Once it is down to temperature, siphon, pitch and ferment.
Counterflow chillers: Immersion chillers are pretty simple; counterflow chillers are not. If you're a biology geek, you might know about countercurrent exchange - counterflow chillers work on the same principal. For those who don't know how that works...I'm sorry, but I cannot explain it properly. Check the countercurrent exchange node, though, to see the biological example of it. Anyhow, counterflow chillers have two inlets and two outlets and keep the wort and chilling water physically separate, but transfer heat between the two. They do it exceedingly well. Problem is, they cost a bunch of dollars. (This is not to say immersion chillers are, 'cheap,' but you can make one yourself for a good price.) If you can score one at a reasonable price, go for it. I don't own one and their operation should be described in the owner's manual.
I covered partial mashing in Homebrewing 205: Mashing and am putting it here for a reason. Compared to moving to all grain brewing, partial mashing is extremely cost-effective in the short run and allows you to expand your knowledge and skills. Compared to going allgrain, partial mashing only puts a small part of your brewing at risk (mainly, not hitting your desired specific gravity perfectly). Switching to partial mashing basically teaches you how to brew all grain, but with less of a risk to your beer than just jumping in feet first. We brewed a half-dozen batches or so using the method, then switched to all grain brewing and didn't skip a beat. Smooth sailing. If you can manage a partial mash batch, you can manage an all grain batch (provided you have the equipment), and don't let anyone tell you otherwise.