Hopefully, everything will go smoothly in the polling places on Election Day. But if you run into trouble, here's some advice: do everything you can (firmly and politely) to make sure you're permitted to cast a regular ballot. Provisional ballots are second-class ballots, and may never get counted.
Provisional ballots are not counted on Election Day. Instead, they're counted afterwards, and are subject to debate and challenge by the members of the Board of Elections. Moreover, they'll only be counted if the number of provisional ballots exceeds the margin of victory for any given race.
So if you find yourself being offered a provisional ballot for some reason, try to talk the precinct judges into giving you a regular ballot. If you're not successful, then before you cast a ballot, go outside and find someone from a Voter Protection group. Seek out someone from a campaign you're partial to (both the McCain and Obama campaigns will have multiple observers and GOTV personnel in place at most polling places). If all else fails, call the Board of Elections for clarification.
Finally, be prepared for a lengthy wait on Tuesday. But remember, so long as you're in line before 7:30 pm, you will be permitted to vote a regular ballot, no matter how long it takes to get through the line. (If a court orders precincts held open longer and you get in line after 7:30, you will cast a provisional ballot that is kept separate from other ballots (even other provisional ballots), so they can be thrown out if the court's decision is subsequently overturned.) So even if you can't get there until after work and the lines are long, don't fear: your vote will count, so long as you're in line at the right precinct by 7:30.