I discovered a few things last year at the South Pole. There are some options that weren't around for the last presidential election, when I received my ballot 2 days too late to vote. Following writeup is oriented towards us, with no incoming/outgoing mail service until late Oct. Anybody who is planning to be on the road in November should look up the Federal write-in ballot.
Washington State for example will cheerfully email or fax me my ballot, which means that all I have to do is get it filled out and in the outgoing mail by Nov 2. Easy! Sounds like many states will do that now.
If you're registered to vote but didn't request an absentee ballot, it looks like most states will let you request one by fax or telephone. States have different deadlines, but generally you should plan to request an absentee ballot before Sept 15.
If you request an absentee ballot but your state doesn't get it to you in time, there will be a Federal write-in ballot that you can download, fill in, and put in the mail. But you have to be signed up for an absentee ballot in your home state to be allowed to use it.
If you're not registered to vote, you need mail service unless you have someone at home with a Power of Attorney. Some states may have special register-by-fax provisions for people like us: doesn't hurt to email your Secretary of State or Dept of Elections and ask. Our taxes pay their salaries, they have people to answer our email.
Links to all the state election information websites
ActivoteAmerica
Nonpartisan "Get out the vote" outfit from Utah. Lots of information and resources. You can find your Senator/Rep's voting record, etc. The "Register to vote online" link isn't really an online deal: it lets you print out mail-in forms for any state. All the different pages on this site play different inspiring melodies, you may want to turn down your speaker volume.
Federal Voting Assistance Program
Includes information on the Federal write-in absentee ballot