CarInsurance.com suggest you follow these guidelines:

• Drive SensiblyAggressive driving (speeding, rapid acceleration and braking) wastes gas. It can lower your gas mileage by 33 percent at highway speeds and by 5 percent around town. Sensible driving is also safer for you and others, so you may save more than gas money.
• Observe the Speed Limit Gas mileage decreases rapidly at speeds above 60 mph. Each 5 mph you drive over 60 mph is like paying an additional $0.10 per gallon for gas. Observing the speed limit is also safer.
• Avoid Excessive IdlingIdling gets 0 miles per gallon. Cars with larger engines typically waste more gas at idle than do cars with smaller engines.
• Use Cruise ControlUsing cruise control on the highway helps you maintain a constant speed and, in most cases, will save gas.
• Use Overdrive GearsWhen you use overdrive gearing, your car's engine speed goes down. This saves gas and reduces engine wear.

source: http://www.fueleconomy.gov/feg/drive.shtml