First best, second best, and third parties
There is a lot of noise about how voting for a third party is "wasting your vote" and taking it away from the only real candidates in this election, those of the two largest parties (The Democrats and the Republicans). Only one person can be elected, and only one other person has a reasonable shot of being elected if the first one isn't.
In some sense, this is true. Gary Johnson (Libertarian), and save us, Jill Stein (Green), are highly unlikely to win. So a vote for them prevents you from voting from the alternative you prefer from the two candidates who might plausibly win.
Leaving aside arguments such as how well Ross Perot, George Wallace, Theodore Roosevelt, and Abraham Lincoln have done historically suggesting there is a possible role for Third Parties in US Presidential Elections, just as there are in Governor races (Jesse Ventura), I want to propose another argument.
The system of two major parties will never be broken if everyone always votes for a candidate from the two major parties. It's basically a mutually-reinforcing belief system. If you believe everyone believes that a third party cannot win, you yourself will believe that a third party cannot win and not vote for a third party in an effort to win. The system cannot change under that logic.
We are stuck in what Richard Lipsey and Kelvin Lancaster dubbed a "second best" solution (the best solution given the imperfections of the rest of the world differs from the best solution if the rest of the world were optimal).
The only way to change beliefs about what is possible in the future is to act differently. If we want what we perceive as a first-best world where the candidate we like might win, we are going to have to vote for those candidates in earlier elections to change the belief structure of other voters about their electability.
Now this election is especially risky (they all are, but this one more so), in that a demagogue is closer to power than usually occurs. However the likelihood of any one vote being the marginal vote is infinitesimally small.
People will trot out the 2000 election and Nader and Gore on the risk of protest votes. Gore lost for lots of reasons, there are many 'butfors', Nader possibly among them, but hardly the only one. (Sighs, Bill Clinton, butterfly ballots, hanging chads, the US Supreme Court, Gore's team asking for a partial but not total recount, and so on).
Yet there are a few million people in this country who think we should have a Green Party government. I think they are wrong. I think Greens should elect some actual Legislators and Senators before they try to run for the Presidency (they do have some Minneapolis City Council members, including my own district). But I also think they should have a voice. And if bless-your-soul, you should think Moscow should run US foreign policy and that vaccines are on net bad, you should vote for Jill Stein (and rethink your life choices). Or even if you think the Greens would be better in the long run, and government should be more green, and want to move the Overton Window in the green direction, you can justify a vote for Stein.
Similarly, this year Gary Johnson is the Libertarian nominee, and he is a more serious threat to the political party establishment, in that he (and his VP) have more governing experience than the major party candidates, and are polling in the neighborhood of 10%. If your first-best world has Libertarian President and Congress (or you simply think we should move farther in the direction of lower-case "l" libertarianism without being absolutely upper-case "L" Libertarian) you should vote for Johnson/Weld. Again, it would be good for the Libertarians to show success at lower levels of government and Congress (beyond Ron Paul) before trying to take over the Executive Branch, but we can't always get what we want.
A serious showing in 2016 helps a third party's candidates in 2020. It reframes people's expectations. It moves towards your first-best world in the long run, even if it is suboptimal from a second-best perspective in the short run. Once a third party gets in the 30% range of support, it becomes a plausible, unwasted vote in the short term. The third party will be unlikely to get 30% support before it gets 20% or 10%. That could all happen in one election cycle, or it could take multiples.
Of course, if you like either the Democratic or Republican nominees best, you should vote for them.
Politics may seem like a one-shot game, but it is in fact repeated. If all goes well, I will spend as much of my life between the years in the next administration between 2017 and 2021 as in the following administration between the years 2021 and 2025. The rules for electing Presidents in the US may be nuts, but they are the only rules we have right now.