The deadline for registering to vote for the November presidential election is October 9, 2012.
If you are a registered voter, even if you have voted many times before, you need to
check your registration before the election: voter purges have happened in multiple states, are ongoing now, and new voter laws in swing states have made formerly allowable voter ID's to no longer be valid for voting.
Check here for the ID's you need to now bring to the polls, and to see if you are still registered to vote:
http://www.canivote.org/
www.gottavote.com
(see "get registered" or "plan to vote" for your state's listing of currently acceptable voter ID's.
If you live in a swing state, especially -
expect to be challenged at the polls day of election. Be sure you bring your correct voter ID to poll with you.
Early voting is available in some states, so you can vote early. See above.