Fill up with a lower-octane gasoline. Buy the lowest grade or octane of gasoline that is appropriate for your car. Unless your car requires premium gasoline, filling up your car with high-octane fuel is a waste of money. That pricey premium fuel won't boost your car's fuel economy or performance in the least, so skip it.
If you're not sure what grade of fuel works best for your car, open up your owner's manual and take a look. As long as your engine doesn't knock or ping when you fuel up with regular unleaded, you're good to drive on this much cheaper gas. Passing on pricey premium gasoline could save you hundreds of dollars a year.