Keep A Fuel Log
Next time you go shopping spend 99p on a small notebook and pen. Keep them in your glovebox and record all your petrol purchases and mileage that you travel between filling up. If you get in the habit of knowing exactly how much money you spend on petrol you become more conscious of how much you are spending. This helps you remember about trying to reduce your petrol costs, as well as showing you how much of your income you spend on gas.
You can also use your fuel log in the future when you come to sell your car. If it proves you get a decent MPG it can definitely help towards the sale.
There isn't any really! Just don't forget to keep track of everything every time you go to the station.
Image credit: sxc / spekulator 2006.