the easiest thing to get would be a MPG meter
http://www.scangauge.com/ it will give you instant feedback on your driving style and how it affects your mpg
you could get a timing chip that improves gas mileage (changes the way fuel is injected, mainly when and such)
some kind of wheel well skirt to reduce drag around the wheels
http://ecomodder.com/forum/showthread.php/aerocivic-how-drop-your-cd-0–34-0-a-290.html
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although I think the solution of “buying more” to improve your MPG isn’t necessarily the best solution
remove any racks accessories that decrease how aerodynamic your car is
remove crap from your car (ie decrease weight)
change your driving style (55mph max)