Lighten your load. Cars with extra weight need more gas. Check your manual to see what type of gas you need. Most cars only need 87 octane, so there's no need for more expensive gas. Don't top off the tank.
Heat can cause the gas to expand and overflow.
The faster you go the more gas you use. Acceleration is a major use of gas. Avoid having to stop and go and accelerate. Don't drive out of your way for cheaper gas. It's not worth the gas to get there.