Reduce your vehicle's weight!
Emptying out the trunk and removing heavy items that you don't need saves fuel. Why are you hauling around things that belong in the storage shed?
For every extra 250 pounds your engine hauls, the car loses about one mile per gallon in fuel economy. Reduce drag. too. Why are you driving around with that ski-, bicycle- or luggage-rack on your roof if you don't need it?
Didn't you know that this increases the wind resistance of your car? Removing such extra items will save you quite a bit of gasoline.