"You have a duty to vote!"
"If you don't vote, you have no right to complain!"
"If you don't vote, your silence is supporting the incumbent!"
George Carlin answers these fallacies quite well here.
Alternatively, "Either your vote is effective, in which case it will be imposed upon others through the initiation of violence, or it is not effective, in which case it is a meaningless waste of time." ~ Riley Yielding
Consider the options you face when you vote: If you are in the majority, you are imposing your candidate upon the minority, and you are forcing those who disagree with you to accept your preferences, which will be enforced at gunpoint. When you are trying to choose the lesser of two evils, you are still choosing evil, and knowingly voting for evil is immoral. There can be no duty imposed upon you by another party in a free society. If someone can impose a duty upon you without your consent, you are a slave. (LT)