The effectiveness of boycotts can be debated. Glenn Beck and Fox News have been hard hit, especially in the UK -- which proves that it never hurts to try.
Low on gas, I risked being stranded the other day to avoid spending money at a BP station. BP also owns AM/PM and ARCO stations so you may want to avoid them if you don't want to inadvertently give them your hard earned dollars.
Planning to change your oil soon? Castrol oil is wholly owned by BP and if you own a boat, BP sells fuel and lubricants for those as well. I'm not a fan of any of the big oil companies, but I have chosen not to spend one thin dime with BP and encourage others to follow suit.