Don't drive aggressively
Rapid acceleration and deceleration drastically reduces fuel economy and costs you big money. According to the DOE, aggressive driving reduces overall mileage by 5% to 33% which is like adding $0.12 to $0.81 per gallon to the price of gas. In addition to the wasted gas money that could be better spent elsewhere, you are causing heavy wear on your vehicle including brakes, tires and drivetrain -- not to mention increased risk of traffic tickets and risk of accidents.
Keep it at 60 m.p.h. or below
The ideal speed to drive will vary according to vehicle. But overall, the ideal speed tend to be between 25 and 60 mph for most vehicles. According to the DOE, each 5 mph you drive over 60 m.p.h. is equivalent to paying an additional $0.24 per gallon! 
Drive like you ride a bike
If you where riding a bike would you race to a red light? Of course not; while on a bike you have a natural tendency to conserve your energy and coast to red lights and temporary obstacles. This same idea of conserving energy can be applied to increasing your gas mileage. The key idea is to get in the habit of looking far ahead and anticipate when you might need to slow down. If you see that a light has changed a full block or two ahead, or if a vehicle has its turn signal on, let up on the accelerator and coast. Every time you are forced to brake, you waste energy and decrease your gas mileage.
Avoid idling for more than a minute
Turn it off or leave it running? The idling time in question will vary by vehicle, but generally speaking time estimates regarding this question and idling time range from about 10 to 30 seconds . As a general rule, if you think you are going to be idling for more than a minute it pays to shut off the engine when it is safe and practical.
Lose the junk in your trunk
The EPA estimates that each additional 100 pound of unnecessary items you store in your vehicle will reduce your mileage by about 1% - 2%; this is like paying an extra 2 to 5 cents per gallon of gas .
Use cruise control on the highway
Maintaining a steady speed will reduce your fuel consumption in most incidences. It may even help you avoid a speeding ticket.
Use overdrive gears
Using overdrive gears allows the motor to work at lower rpm's and thereby increases gas mileage .
AC on vs. windows rolled down
Of course the best mileage will come when your windows are rolled up and the AC is off. However, this is generally not considered practical on hot days. So, when you must choose between windows and AC, the higher-mileage option generally depends on if you're driving on the highway or if your driving in the city. The overwhelming consensus of opinions suggest that:
- On the highway: the best mileage choice is to have the windows rolled up and the AC on. The decreased aerodynamics become much more substantial at higher speed with the windows rolled down.
- In the city: the best mileage choice is to roll the windows down and leave the AC off. At lower speeds aerodynamic drag is much less of an issue.
Check your tire pressure
Make sure your vehicle's tires are not under inflated. Setting your tires to the recommended pressure can increase increase gas mileage by up to 3.3 percent. 
Replace dirty air filters
Changing a dirty air filter can have a significant impact on your vehicles gas mileage. Air filters are often fairly simple to replace. According to one source, replacing a dirty air filter can increase gas mileage by as much as 10%. 
Think ahead; Combine trips; what could save more gas than not driving at all? If you find yourself making to many trips to the store for small 'must have' purchases, take some time to think about personal organization. Nothing beats a good list, and keeping a small list handy in your purse or wallet can be helpful too. Personally, I find that if the list isn't handy, I forget to write things down; this resluts in unnecessary errands. Planning your shopping trips with a neighbor or friend can also be a fun and practical way to share expenses.