I have actually been doing this for a few years now.
It's pretty simple - go next door to BP.
I'll even pay a few more cents for Shell.
You are driving on the highway, and you need to stop for gas.
You see a sign that only has Exxon listed, drive on a few more miles to the next exit.
I'm sure other oil companies have done things that should cause us to boycott them as well, but Exxon has screwed over their own countrymen for 15 years now...
Please read this great article written by my college buddy, Ashley Shelby, about the recent (and infuriating) Exxon-Valdez ruling in the Supreme Court.