I am a certified gasoline waster, but I am trying to change. I drive a 85 Honda CRX and a Chevy Express cargo van. So far I am saving almost 100 bucks a week from changing my driving habits.
Start off with the basics: Take care of your car--make sure it is tuned properly, filters are changed, etc., keep your tires inflated--check the sidewalls for maximum pressure and don't exceed, drive like you have an egg under your right foot, and don't worry about what other drivers think. Do those things first, and then make any modifications that suite your style and your RYI (return on investment).