The above voluminous information, is all true, but most of the effects are pretty small (for example tyre pressure has to be down by a massive 30% to give a 10% increase in fuel consumption), and misses the single biggest, most effective technique of all.
Actually the best way to decrease fuel consumption is sometimes called 'squirt and coast'.
To do this you accelerate using moderate to firm acceleration (choose gears to give about 1/3-1/2 max engine revs or whatever is most efficient for your engine for accelerating) up to a target speed (a speed like 56 mph is OK, but the lower the better due to air drag; 30 mph is better ;-) ). You should try to get about 20-25 miles per gallon while accelerating depending on the engine.
When you reach your target speed you drop the clutch (or put the car into neutral), and coast with the engine idling. You should easily get more than 50 mpg whilst you are coasting, even on a real gas guzzler, probably well over a hundred mpg on an average car.
You should aim to coast for a few times longer than you accelerate for. When you've lost say, 10-15 mph, accelerate back up to your target speed. Lather, rinse, repeat.
Doing this can more than double your miles per gallon, on even an inefficient car; it's by far the most significant thing you can do.
The problem with it is the coasting phase. It will really annoy the person behind you and coasting that way is often illegal and it can be dangerous.
Still, it is very, very effective. Some essentially unmodified road cars with normal engine sizes can achieve a average of well over 120 mpg this way (although the experts often add an ignition kill switch to temporarily kill the engine whilst coasting to achieve this, and then bump start it again; NOT recommended if your car has power assist on the brakes or steering!)
I've used this only once; during a petrol tanker strike in the UK, and there were no almost no cars on the road; it definitely works, my gas guzzling car went much, much further; at least twice as far on the same fuel, probably more (difficult to be more precise when you can't fill up with petrol).
In fact, lordaych's node says that going below your optimum speed wastes fuel. That's not actually true, if you rolled to a complete stop before restarting your engine, you would actually go further, it's just that you gain less if you go much below 30-40 or so miles per hour (particularly if your engine is idling rather than switched off), and it takes you far longer to get to where you want to go. However it *would* be true if you weren't idling your engine- internal combustion engines aren't very efficient at doing that; my car indicates about 50 mpg rolling at 22 miles per hour with the engine out of gear and idling. This implies it gives 100 mpg rolling at 44 miles per hour and so on.
The squirt and coast technique is used for distance racing; internal combustion engines can do >3000 miles on a single gallon, if you keep your top speed under 30mph and have a tremendously lightweight car with good aerodynamics and low friction wheels.
p.s. Oh yeah, the deal with airconditioning- it does use fuel, but you're better off using the airconditioning than opening the window, that increases the drag of the vehicle and usually uses more fuel.
Blondino messaged me that: "the most efficient way of increasing fuel economy is naturally to walk or cycle if you can rather than take the car..." Thanks Blondino, that's deep thinking! I certainly agree that nobody should do any unnatural walking or cycling. However, using a car cannot always be avoided and then these techniques can help.