Drive smoothly

Anticipate stops. Don't run to red lights

Use cruise control. It's possible to "pulse and glide" to get better fuel efficiency but this is really annoying and is more of an esoteric technique. For everyday driving on relatively flat ground cruise control avoids unnecessary accidental acceleration and deceleration which is more efficient. Learn how to use the "resume/accel" and "set/decel" buttons so you don't need to tap the brakes every time you change speeds.

Avoid the A/C. Use the 'fan' and crack your windows every now and then at lower speeds. Or use the A/C, but turn it off when you're cool enough (instead of making the temperature slider warmer).

Get rid of unnecessary weight

Follow at a safe distance - 2-3 seconds behind the next car.

Shift effectively in manual transmissions. Don't run up gears higher than you need to for everyday acceleration. Learn how to ration power to the engine effectively. This gives you more room to beat the EPA estimates that you couldn't with auto

Maintain the car properly. Tire pressure is critical and easy to check.

On the highway, drive at an optimal speed for fuel economy. Usually 50-65mph but really varies a lot by cars.