Vote your conscience
For those who cannot, in good conscience, vote for either Hillary Clinton or Donald Trump (and there are likely to be millions of such voters in November) there are other options.
There is no need to sacrifice the right to vote for other offices on the November ballot. The presidential choice can be left blank, a name can be written in or a third party candidate can be chosen.
Iowa has one of the easiest, perhaps the easiest ballot access rules for third party candidates in the country. In 2012, Iowa had six third party or independent president/vice president tickets on the ballot for the office of president besides the Democrat and Republican tickets.
The Libertarian Party was on the ballot in 48 states and the District of Columbia, the Green Party was on the ballot in 36 states and the District of Columbia and the Constitution Party was on the ballot in 26 states.
There is no reasonable expectation that a third party will win a presidential election.
Because of that fact, we have been conditioned to believe that casting a vote simply as a protest, is irrational and irresponsible, but that choice sends a stronger message than simply staying home.