VOTING IRREGULARITIES: If you see anything wrong, call 1-866-OUR-VOTE. They've got teams of lawyers all across the country, ready to investigate suspected problems immediately. Bring that number with you to the polling place. (Thanks, pir_anha.)
IF YOU'RE NOT PERMITTED TO VOTE: By law, every person who believes they are entitled to vote at a given location must be permitted to vote there. Late in the day, this applies to everyone who is in line to vote before the polls are scheduled to close. If the above applies to you and the election officials won't let you into the voting booth for any reason, they must let you fill out a provisional ballot, which may be counted later after it's reviewed by a judge. If you're not even permitted to fill out a provisional ballot, call 1-866-OUR-VOTE. It also can't hurt at that point to also ask the election officials how you can contact a local election court to get the situation reviewed expediently. Also, many polling places are permitted to require identification. So be prepared. (In Philadelphia, if you haven't ever voted at your polling place, or haven't in the past few years, then you have to bring photo ID; but non-photo ID showing proof of residence, such as a utility bill or your voter registration card, must be accepted by the election judge if it's all you've got.)
Also, you can't vote if you're not registered in most states. (Exceptions may include ID, ME, MN, NH, ND, and WY.) If that's why you're not voting, I'll give you a one-time-only reprieve if you register RIGHT NOW for the next election. (PDF link; you'll want to print pages 4 and 5.)
IF YOU'RE NOT PLANNING TO VOTE (and are a US citizen at least 18 years of age and you haven't already voted in this election): What are you doing today? If that doesn't inspire you to vote, I don't know what will. And if you can vote and don't, then, as is my usual policy, I won't be speaking to you on election day. You have more important things to be doing than talking to me.
WHO SHOULD YOU VOTE FOR? Well, if you know me, you probably know who I'd vote for if I were you. But honestly? It's more important to me that you vote at all. Each of you who reads this probably shares multiple demographic groups with me. The more voters there are who are like me, the more the politicians running for office will care what I think. So if you don't care enough about the issues to vote, then at least vote as a favor to me. Push the "I'm not voting for any candidate" button, if that's what you feel you need to do. But just vote. Be an American Hero. [Link is quasi-NSFW and parts of the site are two years out-of-date, but it's still fun.]
VOTE OR DIE!
Prayer For Voting, by Rabbi David Seidenberg [PDF]