If you have a manual, shift as high as possible to keep your car in a low RPM range. (This can be simulated with an automatic if you know your car well enough to control the shifting via gas flow… but typically automatics are set up for their peak mpg already [ assuming they’re tuned up] )
Go the speed limit (especially on the freeway), the faster you go, the worse your gas mileage (the difference between 65 mph and 75 mph can be as much as 10mpg depending on the car).
Windows up at high speeds (above 45 or so). At those speeds use the fan or the AC (the AC actually uses less gas at those speeds, as the drag created by having the windows down increases exponentially with speed).
Keep your tires inflated to the levels indicated on the tires.
Accelerating quickly will burn excess fuel, don’t do it.
If you’re REALLY pushing for gas, avoid using anything on your car that uses electricity (even the radio). Anything that drains power makes the car work harder to operate the alternator. (but this is a pretty small gain)