1. Check your tire pressure
Under-inflated tires have more rolling resistance, which means your engine has to work harder to keep your car moving. Check your tires when they are cold at least once a month, since driving the car warms up the tires (and the air inside them), which increases pressure and gives a falsely high reading.
2. Check your air filter
A dirty air filter restricts the flow of air into the engine, which harms performance and economy.
3. Slow down and Accelerate with care
As speed increases, fuel economy decreases exponentially. Stick-shifters should shift early to keep the revs down, but don't lug the engine; downshift if you need to accelerate. Keep an eye well down the road for potential slowdowns. If you accelerate to speed then have to brake right away, that's wasted fuel.
4. Hang with the trucks
A constant speed keeps shifting to a minimum as it takes much more fuel to get a vehicle moving than it does to keep it moving. Rolling with the big rigs saves fuel (and aggravation).
5. Get back to nature
Consider shutting off the air conditioner, opening the windows and enjoying the breeze. It may be a tad warmer, but at lower speeds you'll save fuel. That said, at highway speeds the A/C may be more efficient than the wind resistance from open windows and sunroof.
6. Back off the bling
New wheels and tires may look cool, and they can certainly improve handling. But if they are wider than the stock tires, they'll create more rolling resistance and decrease fuel economy. If you upgrade your wheels and tires, keep the old ones. For long road trips, the stock wheels give a smoother ride and better economy.
7. Clean out your car
Periodically go through your car and see what can be tossed out or brought into the house. It doesn't take much to acquire an extra 40 or 50 lbs. of stuff, and the more weight your car has to lug around, the more fuel it burns.