​Make sure your tire pressure is consistent and each tire has the same PSI: having even just a 10% difference can throw the car off enough to use up more gas.

Air filter: Another easy and simple one, this should also be cheap. Think of the air filter as your nose, and you now have a cold. You are all stuffy and backed up, even mouth breathing. You must blow your nose and use some vaporub, aka change the air filter.
Additives: Fuel system cleaners and injector cleaners is where you want to be with additives. These help keep your engine lubricated and your gas moving around freely
This one is ultra-cheap, Forethought! plan your day. If you are going to go on a family adventure, turn it into an activity with the kids. Try to find the most efficient way to get to each destination. Get creative and see how many miles you could shave off your trip in total. I say the one with the best route gets ice cream.
The one that I need to abide by the most is Leadfoot. Ease up on the accelerator. Use motion and energy to your advantage. It is fun to rev the engine a bit and have fun but not when each time you rev it, you could have bought a coffee.
Some honorable mentions would be using your store reward or loyalty program. Some facilities use reward points vs loyalty points. But the idea is still the same, you accrue points for a predetermined discount, and it is also usually per gallon. And if you can pay with cash, it’s usually only a few cents difference, but at the prices, they are now. Every penny counts.