This, I think, has been covered, but with the elections forthcoming and next year being the BIG election for President, I figure it bears repeating.
I was just looking at the US Embassy site for information on registering to vote absentee and found some good information. http://www.usembassy.org.uk/cons_web/acs/scs/voting.htm
One needs to request and then fill in a Federal Post Card Application for a ballot. they also offered this site:
http://www.fvap.gov/ which seemed to be chocked full of good info as well.
I've already done an online request for my FPCA and intend to fully exercise my right to vote!