Hi,
I'm an expert at long-distance relationships. I can tell you all of the best things to do, and what to avoid.
First, if your girlfriend is going to be gone for a year or more, bye bye.
Most human beings are just that, human. We are social creatures. And being apart for an entire year can be devasting. However, in saying this, if she is gone just a few months, then don't worry about it.
I've learned that if you can make regular appointments to communicate with each other, this is a good thing. For example, if she has access to the internet, then make weekly appointments to spend time chatting. Get a web cam and microphone for her in case there isn't one where she is headed.
Send her surprises in the mail. Not email, but real snailmail. Send her pictures, small care and love packages, things she would be surprised to receive. For example, make a home-made card, put a splash of your cologne on it as this will bring back some happy memories.
Use Yahoo's Instant Messenger to leave short voice messages. Just call her on Yahoo YM and you will get her voice mail message. You can check voice mail by clicking on YM's "Contacts" and then click "Voice Messages & Call History." She can do the same for you.
If you have a digital video recorder, or a digi camera that can do short video clips, make some videos for her to watch and burn them on a CD/DVD and mail them to her. I once had a friend who would give me a tour of each of the rooms in her house, even outside, and would narrate them to me. I loved them! As I got to know about her side of the life.
There are really so many things you can do. And you need to do everything. Hopefully she's a wonderful girlfriend and she will gladly wait for you and that the two of you will be together again in the soon future. Pray always, and pray together when you get the chance.