(This is mostly all common sense, but it never hurts to re-iterate it.)
1. Learn about something other than the presidential race! The presidential race is the issue everyone talks about, but in reality, it's the issue that should mean the least to you. Unless you live in a swing state, your individual vote won't make an ounce of difference. (Thanks, electoral college system.) However, your vote does count in other matters. Who is running for Congress or Governor in your state? Learn about these people. What are their opinions, or, more importantly, their political experience. What about ballot initiatives (ie propositions and amendments)? The ballot initiatives are the only means through which you can have a direct voice. When you get to the polls, they will ask you about things like cigarette taxes, partisan/nonpartisan judge assignments, and other weird things written in lawyer-speak. Learn about these ahead of time so that you know whether or not you want to vote for them!
2. Check the facts. There is a lot of false or misleading information out there, and it's your responsibility to know the truth behind them. I recommend factcheck.org. Also, before you go around bashing things like the Affordable Care Act, you probably want to take a look at what's actually in there.
3. Dress appropriately! If you come to the polls wearing a shirt that endorses a particular party/issue/candidate, you will be turned away.
With all that said, my stomach is kind of knotty. I'm keeping my fingers crossed for tomorrow night.