Dumpster Diving Food: 101
How I learned to stop worrying and love the expiration date.

You're poor. You're a college student, or a drug addict, or a squatter, or a crusty anarcho-punk type, or a skinny, quivering, starving whatever; you're YOU. And you're LACKING NUTRITION. You don't have access to food stamps because the shit job you were fired from paid you under the table and you have no proof you ever worked anywhere. Even if you had some stamps, you probably swapped them for alcohol in a back-alley somewhere and are now completely stripped of resources. To compound this conundrum, it isn't even past the first week of the month yet.
Are you going to die??
Are you going to call home to mommy??
Of course not, because you're too clever and too proud for any of that dying or groveling nonsense. Add three cups of desperation to all that and you've got absolutely nothing, because desperation cannot be quantified. Still: the only solution at hand is to go dumpster diving.
Where to go.
Follow your nose, like that tooth-rot-sugar-pushing bird, Toucan Sam. Your local Bakery will not only permeate the air with olfactory delights, but often have dumpsters full of perfectly edible bread after they close. They throw out the bulk of what they make, often before it develops any sort of mold whatsoever. Sometimes, if the rat-god smiles upon you, there'll be individually wrapped loaves of bread, or bags of scones, rolls, or muffins waiting for you right at the top! Other times, you'll find a moldy cookie sponging up a small tide of bleach at the bottom of an empty bin. Learn the trash pickup cycles. In those densely-packed restaurant supply bakery dumpsters, note that they tend to separate daily deposits by a layer of cardboard, and it's usually safest to take stuff in the center, far from the rusty edges/grimy lid/unmentionable bottom.
The general idea is: go where you'd normally go to get what you want. Fighting off scurvy? Grocery stores and local produce markets are good scores for bruised fruits, as long as they don't have a big, nasty trash compactor in the back. Pet pachyderm? Find a local bagged peanut distributor/manufacturer. Need something for that vegan Food Not Bombs potluck? Dumpster your local Whole Foods Market or equivalent expensive health food store for hummus, pitas, and other cruelty-free goodies. Hell, if you can figure out a way to ward off the dogs they keep in back, you might even find some booze at your local liquor warehouse, being that a bunch of unopened bottles got caked up with spilled hooch or something of the like... Let your curiosity and tastebuds guide you. What kind of stuff does that fancy five-star rib house downtown pitch out? What could you dumpster at that big chocolate factory near the traintracks, besides horribly dismembered Oompa Loompas?
Bring a garbage bag and some gloves.
The first blatantly obvious thing you need to be aware of is that this is messy business. Messy not in the "fishing the red flag out of the kiddie pool of pork n' beans Double Dare" way, but messy in the "pulling a container of salsa out of a pool of grimy rainwater and rancid fish sauce" way. Now, don't worry girls, all this stuff is made of space age polymers and vacuum-sealed with dozens of tamper-proof safety features, so odds are, nothing leaked in. The reason you're scoring all this delicious, free food and the yuppies down the block aren't is because they'd rather pay the man rather than go through the minor inconvenience of rinsing clean all the foul-smelling, derelict goods you crammed into a garbage bag in the pitch of night.
Use the trash bag as a liner, if you're on a bike and using a messenger bag to haul stuff around, or just throw it over your shoulder like Santa if you're on foot. They're useful for keeping a car trunk from smelling as well.
Bring a flashlight, Bring a friend! ^_^
Dumpstering food can be lonely business, and much like shopping for new fashions, a second opinion on a questionable item can prevent a major, life-altering calamity a bit up the road. Bring a friend to be a lookout (both for authority figures and your collective health) and help shoulder the load. If you're doing this in the dead of night (of course, if you live in a dense metropolitan area, nobody will care if you do it in broad daylight), a flashlight will be invaluable, as will a pair of bolt cutters in case your target dumpster is chained shut by malevolent managerial forces. Beware however, because that simple cutting action is like eating the Super Mario mushroom that turns you from innocentio, the lowly, garbage-picking trespasser to CRIMINALIO: Arrestable Destructor of Property!!. Be aware. Be aware also that lurking around private property with a pair of bolt cutters and a flashlight is suspicious.
Don't scratch your goddamn nose.
Seriously, you've been elbow-deep in fucking garbage for half the night for crissakes. What the hell are you thinking? Get your finger away from that open mucous membrane!! Wash your hands for once, WITH SOAP. Relax: If you're careful, you won't get any on those lustrously filthy, totally counter-culturally fashionable eco-warrior dreadlocks.
Winter is your friend, summer is your enemy.
Snow, wind-chill and wintery coldness will convert your average metal dumpster into a luxurious ice-box where dozens of local, magnanimous grocers will put aside stores of food for you in these spoil-resistant, pre-refrigerated bins! (Plastic containers of dips, sauces and similar things containing large amounts of water may burst from expansion. These are merely minor sacrifices on your road to nutritional autonomy.) Feel free to laugh at anything with an expiration date / refrigeration advisory as you slip it into your bag, as you are now invulnerable to such whims of temperature.
Pounding heat, insects, and lack of breezes will convert your average metal dumpster into an open latrine of unimaginable rank. Sometimes, everything will be spoiled, burbling, and mixing with something else in a curdling, unpleasant manner. Usually, it just stinks to high hell, and you should generally pass on anything that requires a chilled environment or has a penchant to turn bad with heat.
Dealing with Johnny Law
Unless you live in a rather small town, the 5-0 usually won't get on you for digging in the trash, as long as you're not making an asshole of yourself while you're doing it by generating a lot of ruckus. The underpaid, front-line bakery grunts will, in all likelihood, be unconcerned about your practices and will usually go back to smoking the one-hitter they came out for instead of policing the garbage. In the event of a law enforcement official or a hardass employee, your best bet is to either fall back on ignorance or embrace the loathesomeness of your actions.
You can either:

A: Act dumb. Pretend you didn't know it was illegal (IANAL, but in Chicago, the city owns all the alley trash bins, so technically I believe it is trespassing...) and argue that a garbage picker isn't anything worth their time when all sorts of violent crimes are going on. Or claim you lost something down/around there while casually strolling through the alley (ha! right...), or explain, rationally, that you are looking for cardboard boxes because you're planning on moving soon (this almost always works...)
B: Gross them out. Offer them something to eat, a moldy little Danish or something of the like, then cram something else into your mouth right in front of them and chew it like you're savoring the imaginary salmonella crawling all over it. This works best at Bakery dumpsters, where most of the food is out-of-the-bin edible and unmolested by spoilable fluids, but you can offer containers of food from generally anywhere as a twisted boon to the man...
Either of these courses of action (especially the latter) will clearly communicate that you are a lowly, pathetic, sniveling waste of time and that they should move on. The one thing you should not do under any circumstances is RUN. That is only something an idiot who is doing something illegal would do. The police live for the action-packed chases they see on COPS. You don't want to be chased down like a dog and SEARCHED, now do you? Be confident, and stand your ground. You're not violating any laws, you're not hurting anyone, and you're certainly nor doing anything arrestable. You're just doing your civic duty and easing the garbage man's load to bear.

Hey dumpster diver: How're you gunna get it home?
Cars complicate things. The numbers on license plates can be written down and complaints can be filed, and they're difficult to escape from the law with. They have a tendency of being bright, noisy, and obstructive to alleyways, and can kill dead any argument that you're just some poor degenerate trying to get some food. However, since their capacity for cargo is unmatched by any other mode of transportation, sometimes you just need to use a car to get a massive score. In these instances, they're either best used when parked legally a fair distance away and hauled to discreetly, or when used in a highly-coordinated, lightning fast, multi-person raid.
Bicycles are the better, if not best option for dumpster-to-dumpster transportation; light, sleek, and with the ability to bear at least enough food for a week, a bicycle can be your best friend when there's no one else around to rifle through trash with. A bicycle can be used as a makeshift ladder to get up and around the lip of those really tall dumpsters, and can be thrown up inside one with relative ease in the event that the rider needs to camouflage his or her presence quickly. Let the American Ninja within you shatter out through the pane of glass that is your urban demeanor, and become one with the garbage.
A good, solid messenger bag or backpack can hold enough food for three, and often plastic milk crates attached on the back or a full trash bag secured around the handlebars with a bungee cord will turn your ten speed into a lean, mean food haulin' machine...

Yes, other people in your city dumpster dive too, whether you believe it or not; and often, they're not in your elitist, goofy little clique. Please: Don't tear open all the goddamned bags and leave all the little things your nit-picky little stomach didn't want all over the filthy bottom of the bin; and don't leave *anything* outside the bin, on the ground, as this will attract large, biting, disease-carrying, mammal vermin and fuck everyone else's night up; and for the love of god, don't leave the goddamned lid open when you're done. Seriously. Courtesy is as contagious as that tuberculosis you're going to get by hanging out in all these homeless people places.
Kidding about the tuberculosis! Eat that muffin! Nummy-num!

What do I do with all this food, now that I've gotten it home?
Wash it off. Hopefully, you checked the expiration date/sell by date stamped on any packaged goods tossed out, and you're at least a week inside them. As described in the expiration date writeup by earthquake, things don't abruptly turn into disease-ridden sludge after midnight on the day they expire, but rather, go through a slow process of degeneration. Most stores throw out things that are not expired yet, but will expire sooner than a week of someone purchasing them. Why would someone want to buy a protein shake that will go bad in three days? Herein, you can find your motivation to bolster your diet and bulk your starving ass up: make it your practice to consume everything you get before it expires. If you can't, and don't want to watch a dozen containers of bean dip go bad in your fridge, simply freeze them, and thaw them out when you want to eat them later. This works well with baked goods you want to keep around as well, and I tend to have wonderful results with dumpstered foccacia bread.
On the topic of breads, they're another matter entirely as far as edibility goes. Baked goods are extremely flexible; moldy portions can be cut off, the sole rotten bagel in a tossed sleeve can be amputated like a frostbitten toe, and any other doubts can be resolved by simply toasting the item in question. As the caveman learned, heat is your best bet when you want to be secure in the knowledge that nothing insidious is crawling on your snack. Not 100% sure, since some bacteria has been known to survive nuclear detonations, but secure enough to eat it. A few minutes in a stove or a toaster oven will not only roast off anything harmful, but will also reproduce that fresh-from-the-oven texture and flavor...
Finally, a word of caution.
These are merely amusing suggestions, offered to you by a complete stranger over the internet, for crissakes. I'm not responsible if you get arrested, sick, or sued by the Children's Television Workshop when you haul Oscar the Grouch out of his trash can and roast him on your rooftop or whatever. That said, eat, drink, and be merry.