There are two primary ways to return your ballot:
For Military Voters, note that Military Post can be used both inside and outside the U.S. However, be careful to also note that Military Post can take an average of 13-18 days from combat areas, so be sure to mail your ballot ahead of time if you use this type of Post.
Do not rely on the US Embassy or Consulate to send your ballot back in time. They are not a mailing organization. There may be some assistance through American Citizen Services, however, it varies depending on which country you are in. The best is for you to send your ballot from a post office if you are sure you have enough time for mail delivery, or by an express mail courier (however, that may be costly).
Do NOT give your ballot to another US citizen to take back to the US and drop into the local post. This is a sure way to have it rejected. Overseas ballots must have overseas postmarks on their envelopes! The best and easiest way to ensure your completed ballot is returned in time is to mail it early and ahead of time.