1. Avoid extended warm-ups. Modern engines do not require it. As soon as your vehicle is drivable,  accelerate gently and slowly for the first few blocks until the vehicle is fully warmed.
2. Don't drive fast until the engine has fully warmed to normal  operating temperature.
3. Don't carry unnecessary equipment and minimize drag. Weight is the largest single  factor in fuel usage, and every extra hundred pounds in cargo costs about 1%  in fuel economy. Keep your trunk and backseat clear of any unnecessary items.
4. Avoid unnecessary idling. One minute of idling uses more fuel  than it takes to restart the engine. Prolonged idling uses fuel at the rate of  about ½ gallon per hour. During a six month span in 2005, Safelite Group, an  auto glass service company, decreased gasoline usage in their fleet of vans  and light duty trucks by 6% after implementing a corporate initiative, called  “Turn It Off – Idling Gets You Nowhere.” Remember, when you idle, you get “0” mpg.
5. Avoid sudden stops and starts. Hard acceleration uses up to one  third more fuel. Achieve your desired speed with a steady foot on the  accelerator and try coasting to stop. Speeding, rapid acceleration, and braking can lower your gas mileage by 5% at lower speeds around town, and by 33% at higher highway speeds.
6. Don't vent your anger behind the wheel. Aggressive acceleration wastes fuel and can be dangerous as well.
7. Drive at a steady pace. Plan your route to avoid stop-and-start  conditions and heavy traffic. Be aware of the traffic around you and adjust  your driving to avoid constant acceleration and deceleration.
8. Many traffic light systems are "timed" for a given speed. Try to  pace your speed to make the green lights rather than going faster and stopping  for red or yellow lights.
9. The best fuel economy is obtained at moderate speeds. More fuel  is consumed below 35 mph than at 45 mph, and generally, you'll lose 1 mpg for  every 5 mpg over 50.
10. Try to anticipate traffic jams and avoid them when possible.  Despite stops for traffic signals on other roads, avoiding those expressway  traffic jams can lower fuel consumption as much as 50%. Listen to radio reports for alternate routes around congested areas.
11. Avoid excessive braking. The need for braking can often be  eliminated by downshifting or simply taking your foot off the accelerator pedal.
12. Carpool whenever possible or practical.
13. Combine several short trips into a single trip. Short trips  (fewer than 5 miles) don't let the engine reach its most efficient operating temperature. By combining numerous short trips, you can save on the total  miles driven and take advantage of the vehicle's more efficient warmed-up  condition.
14. If you own more than one vehicle, use the most economical,  especially for commuting or stop-and-go driving.
15. Use the transmission properly. If your vehicle has a manual  transmission, shift gears as soon as the engine can run smoothly in the next  gear. Low gear at 20 mph gives only about two-thirds the mileage as high gear  at the same speed. In second gear, it is four-fifths the mileage you'd get in  high. With an automatic transmission, lifting your foot slightly off the  accelerator will make the transmission shift sooner.
16. When approaching hills, don't wait until the vehicle begins to  "lug" before shifting gears. Don't accelerate once you have started up the  hill, because speed increase is slight and fuel consumption is high. You can  minimize the speed loss by gradually increasing speed as you approach a hill.  
17. If equipped, use the cruise control. A cruise control can gain  1-2 mpg by maintaining a steady, preset speed over any kind of terrain. 
18. Relax while driving. Find a comfortable driving position; fidgeting in the seat leads to constant speed changes and decreases fuel economy.
19. If you drive a manual transmission vehicle, start in second when going downhill.
20. Keep accurate records. Over a period, you can check your fuel  economy; a sudden drop in miles per gallon may mean it's time for a tune-up or other maintenance.
    

* - Source: ProCarCare.com