When in doubt, think about whether your representatives in general are doing the job you think they should. If not, vote against whoever is currently in office. If so, vote for whoever is currently in office.
If you have no idea who to vote for, look at the name that sounds most familiar and vote against them.
Hopefully, you've read any ballot questions, and have thought them over. Hopefully, you've familiarized yourself with the candidates and the issues, but if not, vote anyway. This is your sacred duty as a citizen. Don't just blow it off.
ETA: via ginmar -- For all those American citizens who have died in the conflict in Iraq, vote because they can't.