Washington State makes it really easy for expats to vote. They rolled out the mail-in ballots a few years ago, and I believe the state has now gone exclusively to that system, or at least King County has. When we moved to Canada in 2009, I was able to sign up to have my ballot sent to me electronically. This involves filling out an online form, and then sending back a printout with signature via email, fax or snail mail. And there's a website where you can go to track whether your ballot has been received and signature verified. I've been really impressed with this system, compared to other places where I've voted.