(Note: I'm reposting this so that newcomers will have a chance to see it. Americans living abroad need to register by early October in order to vote. I've added another link to www.voteforchange.com.)
If you're an American citizen living abroad, you have the right to vote. It doesn't matter how long you've lived here, or where your last residence was. (Unless you're a convicted felon, and then some states won't let you vote. But I doubt that applies to you.)
Go to VoteFromAbroad.org and follow the instructions. (If it doesn't work in Safari, use FireFox.) Then print out the form and mail it to the address they provide, which is different for each state.
You don't have to be a member of Democrats Abroad to use this site, though they did create it. They don't keep track of your info unless you join Dems Abroad at the same time, and then only your contact details.
If you are a member of the Armed Forces, you can use this site, even if you live in the U.S.
If you think you are registered, but aren't sure, apply again—you will not get in trouble for trying to re-register, and many states weed out voters who haven't voted in the last two years. You can go to CanIVote.org to find out if you're registered.
Do it soon. The deadlines for each state for applying for a ballot and sending your ballot back are here. (pdf)
If you don't receive your ballot in time, send in a Federal Write-in Ballot. (pdf)
Now, bookmark this page and send it to your friends living abroad.
UPDATE: An even better place to see if you're registered is: www.voteforchange.com.