The Atlantic's Connor Friedersdorf says voters often stay away from third-party candidates because they don't have a chance of winning, but he says there are ways they can show dissatisfaction with Republicans and Democrats.
Postpone your calculated support for someone you don't like until you're standing in the election booth. Before then, support the third-party nominee you'd like to see win. If a pollster asks who you support give their name, not the major-party candidate you may wind up voting for in the end. Doing so doesn't squander your vote on someone who won't win, but could be the difference between a Libertarian or Green Party candidate being included or excluded from TV debates.
Think about whether or not you live in a swing state. If so, maybe it makes more sense to vote Republican or Democrat. But if you live in a state like California, where the Democrat will obviously win, or a state like Utah where the Republican is obviously going to win, your vote is going to have a lot more impact if you're part of a third-party surge that signals disaffection to others.