You simply register to vote at the last voting district you resided in in the USA. Use your current overseas address. It is surprisingly easy, especially if you were already registered to vote there before.
Now whether the politicians for that voting district will listen to you or help you is another question, but that's the same for most voters regardless of where they live.
Usually just federal office holders relative to your last place of residence in the US. That's who you can vote for, anyway. Congressman, Senator, and President.
