The New York Times has a list of "What to Do on Election Day" that includes these items:
5. Know your rights concerning provisional ballots. No voter can be turned away in any state this year without being allowed to vote. If there is a question about your eligibility, you must be allowed to vote on a provisional ballot, the validity of which will be determined later. But if you are entitled to vote on a regular ballot, you should insist on doing so, since a provisional ballot may be disqualified later on a technicality.
6. Know where to turn for help. If you experience problems voting, or if you see anything improper at the polls, you may want to get help. There will be nonpartisan poll monitors at many polling places. (There may also be partisan poll watchers, and it's possible one of them may be the person objecting to your voting.) It is a good idea to bring a cellphone, and phone numbers of nonpartisan hotlines like the Election Protection program's 1-866-OURVOTE and Common Cause's 1-866-MYVOTE1.
7. Be prepared for long lines. In some precincts, the wait may stretch into hours. Try to get to your polling place very early in the morning, or between the before-work and after-work rushes. As long as you are in line before the polls close, you are legally entitled to vote. Do not let poll workers close the polls until you have voted.
Here in Chicago, the U.S. attorney's office will have a hot line to report voting-related complaints. The number is 312-469-6157.