1. Keep your tires properly inflated.
Buy a quality tire gauge and check the pressure of your tires before you start.... remember to check while they're cold and do it at least once a month. When your tires are under-inflated, they require much more horsepower to rotate, thus consuming more gas. Most cars have a label that lists proper tire pressure, usually on a plate attached to the drivers door. Your owner's manual has the original tire specifications and required inflation pressures also, as long as you haven't changed tire sizes, these are the numbers you want to target.

2. Lighten up and don't haul anything you don't absolutely need, around with you.
Check your trunk, glove box and front and back seats for belongings that you really don't need on a permanent basis. This won't save you a fortune (unless you have a habit of driving with the full trunk all the time) - but with gas prices headed closer to the $4.00 mark, it does save enough to consider an automotive clean out, and it doesn't cost a dime.

3. When old man winter coats your car with snow and ice, try to remove as much of is as you can, don't just clear a hole in the windshield.
Snow and ice add significant weight to your car, they also increase aerodynamic drag dramatically... which burns even more gas. As a side benefit, clean clear windows improve your ability to see, which improves your margin of safety in dangerous winter driving.

4. Remove bicycle or ski racks in between trips.
It's not really the extra weight that hurts your gas mileage; it's mostly aerodynamic drag.

5. Don't fly flags on your car.
And don't fly flags outside your car's windows. Yep, your guess is correct – it's aerodynamic drag we are talking about here... and your gas bill.

6. Do not fill your tank up completely.
Instead, keep it half full. Depending on your tank size, your car will have 50-100 pounds less to haul all the time... less weight, less gas.

7. Fill your tank at the coolest time of day.
Fuel is denser when it's cool in the early morning or late night.. Your engine consumes fuel by weight but gas pumps dispense fuel by volume. The colder the fuel is when you pump it, the more of it you get for the same money.

8. Try not to stomp on the gas anymore than you need to.
Aggressive acceleration equals maximum gas consumption. The slower you accelerate, the better your gas mileage will be. On the other hand, if you creep along like a snail, the drivers behind you will get mad. Experiment with how little "pedal" your car needs to move at a reasonable traffic speed and save your gas.

9. Likewise, try not to slam on your brakes.
The more you brake, the more you have to accelerate afterwards, and that costs money. Accelerate smoothly and brake soothly. Ideally you want to accelerate once, and then drive at a constant speed until you arrive at your destination. There are too many moving pieces to get stopping and starting patterns right every time, but the closer you get to constant speeds, the more gas you will save.

10. Use the landscape to your advantage.
If the road goes up and down, don't try to maintain a constant speed. Let your car accelerate down the hill, so its inertia will help it climb up the next hill, and let the speed decrease slightly while you are going up. Of course you have to coordinate this with the traffic flow.

11. If you have several cars, use the one with the best gas mileage for daily commuting.
That car is usually easier to park, too.

12. Plan your route to avoid traffic jams.
Because you can't avoid excessive idling, braking and acceleration while in a traffic, traffic is usually responsible for a big chunk of your gas consumed. You might avoid jams sometimes however, if you learn traffic patterns in your area and use them to your advantage.