There are lots of things you can do for free, but until you get a Scangauge (or other MPG instrumentation), you will be learning slowly, on a tank-by-tank basis.
The free stuff:
- stop idling (key-off at stoplights)
- lower your average speed (serious attitude adjustment!)
- take a better route (45mph with no stops is ideal), anticipate traffic lights & slow down early so you don't have to stop
- keep your tire pressure up to sidewall max, at least
- reduce short trips (your scangauge will show you how much fuel it costs to warm up your car)
- pull the Air Conditioner fuse so it doesn't engage the compressor when you hit the defrost
Almost free:
- kill switch for engine-off coasting without using the key
- grill block
- remove engine accessories (who needs power steering? pull the pump & drain the rack & get a shorter accessory belt).
- Mirror delete
- smooth wheel covers.
Good luck!