Public request: Please don’t vote your conscience. Your conscience means nothing to the rest of the world. Your actions, and their consequences, are what matter. True morality is concerned with others, and results affect them, motivations do not.
I’m not telling anyone how to vote (or not), I’m just telling you why to vote (or not). You make your choice based up a realistic expectation of what you think will bring about the most good and the least harm for society as a whole (if you vote in your own self-interest you are being selfish, pretty much by definition). It takes a fair amount of understanding of the political system to do that. It takes a careful analysis of the past and real knowledge of not only what positions different candidates take, but what their possibilities of implementation are. There isn’t always a good answer. There definitely isn’t a quick and easy answer. If your only goal is to feel morally comfortable with your choice, chances are you’re not looking hard enough. Politics is ugly but it does make a difference in people’s lives – more so for those who have the least ability to influence it. Ask yourself some tough questions and be honest in the answers.
If you want to send a message, will anyone hear it? Is it realistic to expect your message to make a change in the world? What's likely to happen in the meantime? A quick look at history suggests that any messages sent by ballot have gotten lost - the only thing people remember in an election is who won and who lost.
The policy positions, the moral character, and the good intentions of the loser don't amount to anything. Voting for who you think will make the best President only matters if that person has a realistic chance to win. If you value voting for the best person, chances are we could all come up with someone we think is better than any candidate on the ballot, but we don't vote for our intelligent and kind cousin because even though she might make a perfect president, we know such a vote is wasted because she will not win. It's the moral equivalent of not voting, which simply lets others decide who gets to run things.
Does your vote support a party or group you want to benefit? Again, in our system, especially at the Presidential level, winning is the only benefit of an election. Voting for a candidate sure to lose does little to support their party. Donating money or time, voicing your support in public forums, expanding their reach and exposure all are good ways to help a party, but a throwaway vote does little.
If your goal in voting is to allow yourself to sleep well at night, to feel comfortable that you stayed above the fray and avoided the messy and imperfect choices that democracy delivers, you are being selfish. On the other hand, if you think your actions, however imperfect and regardless of the outcome, had a good chance at creating a better, more just world, then you did the right thing. Accepting reality is the first step in being able to improve it.