Besides driving habits, I think these things will help:
-install a CAI (I noticed about 1-2mpg)
-remove excess weight (bowling ball, weight set...etc)
-use "v-power" Shell gas or a gas that has no ethanol (I noticed about 9% increase in number of miles I got out of 12.5 gallons of gas, which is when I usually fill up. I got 310 miles instead of 280 miles)
-Install engine management system (Ive heard up to 3mpg increase when this is installed and your car is tuned on a dyno)
-use synthetic oil (I have no experience with this, but people have suggested it helps)