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Don't warm up your engine. Today's vehicles are designed to run from immediate start-up. Warming up the engine just wastes gas.
Don't start and stop your engine needlessly. Idling your engine for shorter periods of time (less than one minute) will not waste gas.
Revving your engine wastes gas and is not needed.
Eliminate fast starts. Rather, accelerate slowly from a stop.
Coast your vehicle to approach stop lights and stop signs rather than accelerate aggressively and then brake aggressively.
Slow down -- traveling at 55 MPH consumes 20% less fuel than 65 MPH.
Keep your tires inflated properly. Poor tire inflation can lead to decreased fuel economy from additional work the engine will have to do.
Make sure your windows are rolled up while traveling at highway speeds. This decreased drag forces on your vehicle.
Running the air conditioner is OK -- you need to be comfortable, and running the A/C only decreases fuel economy by 1 MPG per tank on average.
Drive at a constant speed when possible. Constant speeding up and slowing down wastes fuel.