1. Properly inflated tires. For gas mileage purposes I run my tires on the higher side of the PSI specs. This is probably the most important thing you can keep up with. Tires lose about 1psi per month. If you haven't checked your pressure in a year it could be down 12psi, and only God knows what PSI it was at to start with anyways. Imagen riding with the brakes slightly on everywhere you go. That's what it's like with low tires. The correct tire height is also key.
2. Being consistant with your throttle makes all the difference. Easing off the line, steady speeds, and driving moderately will give you better mpgs. If you have cruise control use it! Drivers that speed up and down get bad mileage.
3. Keep a clean car. Every little bit helps. In the long run wind resistance will add up to a loss in MPG's. If your car is waxed it will especially help on long road trips. (There's a reason why Nascar waxes everything, even windshields, not just to look clean)
4. A lower car will get better mpgs at speed. A lower car creates less drag than a high one. This will add up to more mpgs. I noticed quite a big difference when I had my stock 06WRX compare to a stock 06WRX dropped alot on coilovers. You could only imagen the difference in a stock truck to one with a 12" lift.
5. Fresh oil changes. You should already be staying on top of your oil changes. Just remember this makes a huge difference on the performance of your car. There's products on the market specifically for increased MPGs also. I used a Mobil 1 synthetic that was made to improve MPG's. I believe ENEOS also has synthetic for Economy cars.
6. Not over modding. There's performance mods that will increase MPGs, but there's a certain point where it will start taking away from your good gas mileage. I've always noticed a little better mileage from a Cold air intake and exhaust. I noticed with a bigger throttle body that the mileage went down some.
7. Keep a light load. Don't let your car fill up inside with excess stuff. If your trunk has some extra shoes, clothes, books, etc... all that adds up to more weight and less mpgs. I notice a difference when I come up to the bay area from LA with no extra stuff from when I come back around christmas with a trunk filled with gifts. Weight takes more power to move.
8. Fuel efficient products. Do some research on what is out there that saves MPGs. I already mentioned oils that increase mileage. There is also electronic devices that steady the spark of your ignition or create a more steady amp output. There's also oil and gas additives that increase MPGs. I know there is fuel additives that are proven to work. You have to figure if spending $3-4 bucks a tank is worth it. It would be if you saved $6 dollars in gas at the end of the tank.