Here are specific suggestions from ASE:

• Slow down. Speeding and hard accelerations waste gasoline. Use cruise-control on highways to maintain a steady pace. When waiting for friends or family, shut off the engine. Consolidate daily errands.
• Lighten up. Remove unnecessary items from the vehicle to reduce weight.
• Don't pollute. Dispose of used motor oil, antifreeze/coolant, and old batteries properly. Some repair facilities accept these items. Or call your local government. Keep the engine running at its peak-a misfiring spark plug can reduce fuel efficiency as much as 30%. Replace filters and fluids as recommended by the owner's manual.
• Get pumped. Keep the tires properly inflated and aligned. Under-inflated tires waste fuel by forcing the vehicle's engine to work harder. Moreover, properly maintained tires last longer, saving you money and lessening the burden at landfills.
• Know your limitations. If you are not a do-it-yourselfer, find a good technician. Ask friends for recommendations. Check the reputation of the repair shop with your local consumer group. Check out the technicians' credentials. ASE-certified professionals have passed one or more national exams in specialties such as engine performance and air conditioning.
• Don't try this at home. Your car's air conditioner should be serviced only by a technician certified to handle and recycle refrigerants. The air conditioners in older vehicles contain ozone-depleting chemicals, which can be released into the atmosphere through improper service.
• Instant karma. In addition to helping the environment, routine maintenance will help your vehicle last longer, get better gas mileage, and command a higher resale price. Pretty cool.

Article from the National Institute for Automotive Service Excellence.