All American citizens can vote, regardless of where they live. This year, turnout is likely to be low, so every vote matters.
But if you live overseas like me, you must register and request your ballot every year. You do this with a Federal Post Card Application (really a sheet of paper), but each state has its own rules and deadlines, so the application process can get complex.
There's a simple solution: Vote From Abroad
This website will ask you a few questions and create the proper application ready to be printed out, signed, and mailed. It also provides the address you should send it to.
If you don't get your ballot in time, the site can also provide an emergency write-in ballot.
Full disclosure: This site is provided by Democrats Abroad as a public service. By law its service must be non-partisan, and it discards the personal data you enter as soon as your forms are produced. The only goal – the obsession, in fact – is to help as many people vote as possible.
The website answers frequently asked questions about voting: https://www.votefromabroad.org/faq-page
It also has a list of deadlines for requesting a ballot and returning the ballot:
Where on Earth will you vote in 2014?
— Sue Burke