I agree with a lot of what CheeseBiscuit said.
As for me, personally, because were both military, communication wasn't always possible 24/7. There would be weeks when I wouldn't even hear from him due to his ship cutting communication from the rest of the world. We couldn't text each other as frequently, either, as there's no signal inside the ship. We did email as often as we could, text, MSN, and phone calls.
When I was at school in London, and he in Portsmouth, it was the same thing. Texting, emailing, and calling. We were usually able to see one another on the weekend, unless he had watch duties.
When he was deployed during the first part of our marriage we would email everyday, he would do his best to call me twice a day- but signal wasn't always possible, and again, communications would be cut off, sometimes texting if he was docked somewhere, and letters! lots of letters and cutesy drawings! Just as Cheesebiscuilt said, we both seem to be connected mentally as well.
Granted, I'm in a much different position than most, but communication is always always the key to maintaining a healthy relationship. It also helps that we're both very independent people.