Modify driving habits
Single most significant difference, in any vehicle: "Adjust the nut behind the wheel"
(Drive more efficiently.)
Stay at (or below) the speed limit.
(Remember, you only save 7 seconds per mile going 75mph instead of 65mph, but stopping distance and wind resistance increase exponentially; twice as fast = 4 times the force)
(I have a much more in-depth look at speed at ecomodder.com/blog/slow
Accelerate gently (Keep RPMs low whenever possible)Never
accelerate towards a stop light. (Every time you brake you are wasting momentum. Remember this with the rhyme: "if you have to break, you made a mistake)
Never idle. A popular misconception is that it uses more gas to start than to idle. This was true of older (carbureted) engines. This is not true of modern (fuel injected) engines.
More advanced techniques include actually shutting off the engine and coasting as often as practical. (This can change the feel of the brakes and steering, see step 4)(PLEASE READ the entire instructable and all the comments before commenting about how dangerous it is to coast.)
NOTE: many automatic transmissions can be damaged by coasting with the engine off.
To find out if yours is one of them, consult this list: http://www.motorhomemagazine.com/dinghytowingguide/
If you have an automatic, and your car is on the list, you can shut the engine while moving.
If it is not, do so at your own risk. You may still benefit from shifting into neutral (engine on) when coasting downhill or towards a stop, depending on whether or not your car has automatic deceleration fuel cut off (DFCI) built in (many newer cars do).