If you're serving in the Armed Forces you may find it difficult to register to vote or obtain an absentee ballot, even if you are stationed in the US. Visit http://www.fvap.gov or http://www.overseasvotefoundation.org to request help with absentee voting if you are overseas.
You may find trouble establishing residency in the state where you are stationed, but this shouldn't keep you from voting. You can always vote absentee in the last state in which you were a resident. You can usually request an absentee ballot from your state's Board of Elections or Secretary of State. Check here to learn more.