I don't vote. In the American system, voting is essentially a meaningless act as all but a few states are going to reliably go for one or the other mainstream parties. Furthermore, I don't vote because I believe that in the act of voting I am compromising my principles by lending legitimacy to a system that I disagree with. Fundamentally, I don't agree with democracy, except in some limited forms, but I particularly don't agree with the way things are run in the U.S. and in the two-party system. On referendums and elections for town mayors, etc. sometimes casting a vote is worthwhile, but generally is not. I was a registered Republican and perhaps still am although I think it may lapse, I also participated in voter mobilization stuff as VP of our College Republicans chapter, yes, basically manipulation and encouraging people to vote who might not otherwise but who would reliably vote Republican, mainly rednecks out in the woods and stuff, but that was mainly about local politics; upstate N.Y. tends to run quite conservative but as a state due to population imbalances in the urban areas (not just, but most importantly, NYC) it is reliably Democratic in national elections, thus a vote for or against is worthless.
And again, I believe that my abstinence from voting is a small political (or anti-political) act in itself, and I share this with people when political discussions come up and sometimes it leads to opportunities to share my views and correct some people's perceptions of voting as a civic duty, etc. As someone said, "don't vote; it only encourages them." I'm a believer in that.