Main thing you can do is stay up on maintenance. Air filters, fuel filters, tire pressure, alignment, etc.
In terms of aftermarket, I myself have added a cold air intake with a K&N cone, a throttle body space from Helix called the "Power Tower", which has these scallops that supposedly spin the air entering your plenum. Makes a pretty cool whistle sound between 1500 and 2200 RPM, almost like boost! MPG wise, if I'm on the freeway cruise control at 60 mph, I've gotten up to 23- 24 on flat ground. I drive a 2001 limited.
I also have new performance wires and brand new Densos.
If you have everything dialed, you can get that MPG up, but don't expect extraordinary gains. If you're at all aggressive, kiss those goodbye! I get about 17-18 average, minimal hossin around.
You'd swear it was a V8 by the sound after the two mods put together! Pretty inexpensive too.
Cheers