Here are my suggestions:
-Keep your tire pressure up. Most tires are rated to a maximum of 40PSI. For every 10* the air temp rises, your tires will increase 1PSI. Figure out the highest temp your area hits on average, then figure out the current temp when you fill your tires vs that highest temp. Here in Missouri around 70* air temp I fill my tires to 37PSI. At 100*F air temp, my tires will be right around 40PSI.
-Change your spark plugs on schedule. Copper plugs need to be changed every 15k miles or so. Iridiums, platinums, etc all have higher change rates but don't work as well in some engines.
- Change your fuel filter every 50k miles.
-Add a good fuel system cleaner (SeaFoam is one of the best and a bottle should be good for 2 fill ups) every few fill ups. At least once a month is a good idea.
-Change your oil on schedule. Every 3 month or 3,000 miles, whichever comes first. Some synthetic oils say you can run them for 5k-7k miles but I still recommend oil changes as close to 3k/3 months as possible.
-If you drive an automatic, use the overdrive gear if one is available. This will allow you to enter a "cruise" gear when you reach the appropriate speed (generally on the highway). There's very little power but enough to keep you moving at highway speeds.
-Avoid stop and go traffic. If you can find a route that isn't going to go way out of your way but takes highways most of the way, go that way. Don't speed either. If you can maintain 55-65mph, you'll get your optimum fuel mileage.