Ok, I’ll just write in.......
Can’t stand McCain but don’t want to help Barack Hussein Obama, Jr., the charismatic, messianic Magic Negro, either.
Study the field. Whoever is over to the left of Barack, be it Ralph Nader or any of a number of other really fever-swamp left winger radicals, select one and give him (or her) your wholehearted endorsement. Take no halfway measures here. The goal is to deprive Obama of at least as much support as is being deprived of John McCain.
To vote for Bob Barr as a protest against McCain, is to essentially GIVE one-half a vote to Obama. But if the shift is made to vote the other side of the spectrum (in just this one race, for the Presidency), this destroys a little bit of the plurality that Obama would pick up, and deprives them both of half a vote.
Of course, you may still split the rest of your ticket for the INDIVIDUAL candidate whom you consider best for the job at hand. Sometimes, it won’t be the Republican.