These relationships are difficult, and you have to accept from the start that the situation isn't ideal. On the other hand, a few people actually prefer such relationships, and there are some advantages, eg you can't really annoy each other if you are mostly apart, and the romance and excitement is stronger for the same reason. So you should enjoy the positive aspects as well as being realistic about the negatives.

Is this situation short term or long term? Long term is more difficult of course. In that case you need to set aside time, ideally every day, for contact with each other. Short texts are good, and best is probably Skype. It sounds silly but you should make sure you have a really good webcam and sound equipment, as it's really comforting if you can see and hear the person as if they were in the room.

You should also write - not just emails but sometimes a card or letter, it's something nice to keep. And before you have a conversation, think of important things you want to say, as your talking time is limited. Above all, try and arrange to meet as often as you can.