First if you didn't already read the thread
100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com. That has everything from the mild to wild fe improving ideas. IMHO, when starting focus on driving techniques initially, because they're basically free. The only exception to this is basic maintenance, if your car has never had a tune-up and your tire pressure hasn't been checked since the tires were installed you should get onto those pronto. Also, keep in mind that just because somebody got great results out of a mod doesn't mean you will. Some mods give better results with certain vehicles or driving situations(e.g. aero mods will give better results if you do a lot of highway than city.)