Don't forget the quality of the gas you buy. I've seen a 20% swing in gas mileage because of gas. The first and best way to get better fuel economy is to avoid ANY alcohol blended fuel.
Next you have to train yourself to be light on the entir. I play a game of trying to be the last person at the next light. That means I've wasted no gas and no time

. You can get a Scanguage for $160 that will give you an mpg readout or a $30 vacuum gauge to show engine load, but whatever you do you need to train yourself to be light on the gas to get up to a 30% improvement in mileage.
Do the speed limit! No kidding. You'll get 1 to 3 more mpg just by doing the speed limit.
Air up your tires. (WARNING! this can increase braking distance and lower traction in wet conditions) I put 40 psi in my tires. This will reduce rolling resistance and add mpg. It will also increase tire life because your tires will run cooler. The only other downside is a less comfortable ride.
EDIT:
A couple of more things:
Get your battery checked by a place that has the advanced model. Our Autozone has one and they do it for FREE. It advanced battery & charging system checker should test each cell voltage. A battery with a dead cell will load your alternator and kill your gas mileage.
Additional loads are the A/C and the car electronics. Keep them to a minimun. Plan trips so you don't have to drive in the heat of the day.