Or you can vote third party.
As a Libertarian, I spent many years preaching that people should vote for a third party. Over time, I started to realize that it wasn't really so much of a social problem as a technical problem. Specifically, plurality voting has a known weakness, and it is gamed by considering only the two most-likely parties, and picking among only them. In other words, even if you manage to bring a third party into popularity, plurality voting will soon "fix" the situation until only two dominant parties remain.
So, the answer, it turns out, is not to try to bring a third party into popularity. It is to pick one of the parties and work to reform it. Yeah, I know, it sounds imppossible, but hey, it's more possible than bringing a third party into popularity (without revising the constitution). You really do have more sway in the primaries than in the main election anyway. So, pick one of the big two, and get active in their primaries. Then don't even waste your time voting among the final two contenders--you cannot make a difference there.