Be sure you're registered to vote--even if you're sure you're registered to vote.
CBS News reports mistaken, human-error-based, or just plain illegal purging of voter rolls (the voter rolls cannot be altered within 90 days of a Federal election, yet are continuing to be purged as I type this). Several of the states engaging in the illegal roll purging are battleground states. Shocking, I know. We're talking tens, if not hundreds, of thousands of eligible voters who will show up at the polls and find they are not registered.
Here is a summary of the Voter Purges Report from the non-partisan Brennan Center for Justice at NYU's School of Law. (You can also download the full report in .pdf format.)
Here are some other ways battleground states are making sure that many Democrats citizens are ineligible to vote.
Check your registration status here or here.
How come this isn't all over the news? And even more (pathetically) shocking, when did CBS grow a pair?
ETA: pun comments below: "In a federal election, every voter has the right to fill out a provisional ballot. Anyone who shows up at the polls and is told s/he is not eligible to vote should demand their right to a provisional ballot and continue to contest the issue until his/her ballot is counted." Should have included that before.