Tire pressure, I cannot stress enough the correct tire pressure to save gas. The lower the pressure from normal the more it takes to rotate the tire causing more gas to be used. Along with wheel alignment and equally rotated tires these can all help save you some.
Some tips besides the main categories above include using cruise control on the highways to give a constant even flow to the engine instead of up and down pedaling. It helps save the engine, gears (transmission parts wear), and gas.
Combine errands in one orchestrated trip, don't waste gas going to 14 places if it is all in one store. One thing I never get is people driving around for a good price on cigarettes, you waste the cost driving and time and wear on your car every-time you refuse to pay 50 cents more, a mute point.
Don't rev the engine, as fun as it is, it uses gas obviously
Try not to use air conditioning if at all possible, it uses gas
Tune ups and maintenance keep the engine running at normal, neglect forces the engine to work harder and use more fuel to get the same work done.
Fill tank during cool hours, stops evaporation