Smoothly does it
Studies have shown that an aggressive driving style characterized by sharp acceleration, high speeds, and hard breaking reduces travel time by only 4% on average, while increasing fuel consumption by up to 40%. Avoid unnecessarily marked changes of speed by keeping your distance from the vehicle in front, slowing down gradually when approaching junctions and red lights, and keeping passing to a minimum. Try driving in soft-soled shoes—this should help you to really feel the pedals.

I changed my driving this way, and it actually reduced my gas consumption by about 30%!

Slow down!
Faster driving uses more fuel, so try to exert gentle pressure on the accelerator and keep speeds down to maximize your mpg.

Driving at 50mph uses 30% less fuel than driving at 70mph.

Lay off the A/C,
because using it can increase fuel consumption b more than 20% in city driving. Unless it’s stiflingly hot, roll down the windows instead. If you’re traveling faster than 45mph, it’s more efficient to use your car’s flow-through ventilation than to roll down the windows, because at higher speeds open windows increase the drag on the car.

Get your car serviced regularly
This should pay for itself in saved fuel. An inefficient, poorly maintained engine can reduce your car’s fuel efficiency by 10% or more.

Avoid excess weight or drag
[I]t costs you fuel. Leave the luggage rack behind, and take heavy items (golf clubs, etc.) out of the trunk if you’re not using them. Carrying an extra 100 lb. can reduce fuel efficiency by nearly 2%.

Use the right gear
If you’ve got a gearshift car; move up to top gear as soon as possible without accelerating harder than necessary. However, don’t let the engine labor in high gear when traveling uphill, as this will use more fuel and put stress on the engine. If you’ve got an automatic, ease back slightly on the accelerator as the car gathers momentum, so that the transmission can shift up quickly and smoothly.

How to Reduce Your Carbon Footprint, by Joanna Yarrow. pg. 111-12.