The plan was taken from the Simple Dollar blog, and here's how it works:
When you go to the pump, buy gas as normal and make sure to save your receipt.
At home, designate an envelope for your gas savings. Grab a calculator and multiply the number of gallons of gas you bought by $4. That's the number you would have spent if gas was $4 per gallon.
Take that number and subtract what you actually spent at the pump. You can round to the nearest dollar and put that in your envelope of savings.
Your savings will vary each time you fill up, but you will never experience sudden sticker shock when gas prices peak.
Depending on your commute, this could put hundreds of dollars back in your pocket after a year of saving.