I want to open with a small note on turnout. In 2006, based on this, a house race in 2006 was decided by an average margin of just 14730 votes. When it comes to senate or presidential races, with vote counts in the millions, it is easy to lose sight of the fact that fact. Why is that significant? Your hometown, or your city ward, or your neighborhood community probably has that many people. In an off year election with voter turnout in the 40% range, it would only take three or four neighborhoods turning out en-masse to swing an 'average' house seat. So yes, each vote counts.
What if both candidates make you sick to your stomach? What if you feel like you're stuck between choosing a child molester and a drug dealer? Vote anyway. Pick a minor party, greens, labor, working families, even the constitution party if you like.
I'll give you three good reasons why that you may not have thought of below the fold:
You know that saying about democracy? "The politicians are held accountable to the voters every few years"
Let me parse that sentence to an equivalent: Politicians will not be held accountable to those who don't vote, every two years. Put bluntly, if you don't vote, no politician has any reason to listen to you.
What about the possibility of dangling your vote out as some kind of a reward for pleasing you? Also going to be a no-go. Let's apply some game theory to the effectiveness of the 'vote as a reward approach. You and a reliable voter go to a politician. You say "do X or I won't vote" and the other person says "do the opposite of X and I may vote for you; do X and I vote for your opponent". Now let's sum up the expected vote values to see what the politician will do:
You are an unreliable voter. All things being equal, you will vote 50% of the time.
The other person is a reliable voter, so P(vote) = 1. But P(reward for going ~x) is 0.5.
If the politician does X, the politician gets a net average of -1.0 votes, vs +0.0 votes if the politician does the opposite of X. That is a 1.0 vote penalty against doing X. Now we sneak in the effects of a voting for a minor party. Suddenly, if your vote probability becomes 1, the equation changes. Now, doing X generates 0 votes, and doing the opposite of X generates 0.5 vote. Why is that significant? Your position just cut the penalty for doing what you want in half.
Finally, going back to the 2006 race, the average congressional district had 180,000 votes. Third parties received an average of 3678 votes in each district; a drop in the bucket. But an 11% increase, or 20,000 votes for a third party, would have a huge impact. Imagine if the green party consistently got 20,000 votes, or 10% of the vote share. Suddenly the greens become a power broker, and a pool of votes to be sought after. Within four years, candidates from both major parties would be looking for ways to appeal to those consistent voters who vote third party as a protest vote. Suddenly a few people who decided to 'throwaway vote' changed the rhetoric in a race.
Even 'throwing your vote away' to a third party gives you more power than staying home. The only choice that is unacceptable is to decline to vote.