You have to buy gas, but you don't have to buy newspapers, milk, candy, etc. Go buy your gas and boycott any other purchase. You can get everything you need at the grocery store. How long would it take for them to feel it, and to get the message?
Probably quite a while. Most stations are still independently owned and operated, and they depend on those other sales for (extra) profit. The oil companies will rake it in whether you buy Milk Duds or not.
What desperately needs to happen - particularly up here in Soviet Canuckistan - is for the taxes to come off gasoline. The price of gas could be slashed basically in half if the government would reduce/repeal the gas taxes. (And it's not like they aren't raping us enough everywhere else!) Unfortunately, it's such as cash cow that it won't happen short of an armed revolt.