After roughly a 2 year long distance relationship (courtesy of me joining the military, dealing with tech school, etc while the grilfriend - then fiancee - was in college). . . the key is the following.
Communication
Use up those cell phone hours, as that's all you're going to have for however long you are away. Sure e-mail, IM, the like, are nice. . . but chicks get tired of e-mails and IMs and want to hear your voice. Sounds cheesy, but we all know how emotional females can get.
Nevertheless, make sure your communication is up to par, if not better. Any spare time you have, take a return trip back home and spend a couple days with the significant other. I know it can be tough, and not always accomplished, so your travels may vary.
I wish you luck!
edit: Ah snap. . . didn't even notice usmc-sgt had stated this already. doh!