Whether you were expecting it, or it's something that has recently been put on your plate, having your boyfriend go overseas can be heartbreaking; it doesn't matter if you have been together three weeks or three years. Give your boyfriend a gift before he leaves to show him what he means to you and how much he will be missed.
Buy two copies of the same book, with the instruction that you and your boyfriend both must read a chapter or two a week. Pick a genre that you are both interested in. At the end of each week, discuss what you have read. Video chat book club meetings are now in session! However, if he (or you) does not have a webcam, e-mail, texting or plain old telephone calls are other ways to discuss the novel.
There's nothing more personal and romantic than sleeping in a piece of your significant others' clothing. If your boyfriend is going overseas, buy a T-shirt in his size, sleep in it for a few nights and present it to him right before he leaves. With any luck, it will smell like you, and whenever he wears it, he will be reminded of you and what is waiting for him when he returns home.
In addition to the standard photo you need to give him so that he's got something to show off to his buddies, a super cute idea is to take a few photos of small sections of your body. When he puts together all the little photos in the right order, he will have a large scale photo of you. Make sure to warn him ahead of time that this jigsaw puzzle is for his eyes only.
Keep it simple and write your boyfriend a love letter. Sometimes an old classic is the best gift of all. Writing down on paper how much you love him and how proud you are of him is a gift that is sure to touch his heart; and is one that he can pull out and reread whenever he needs an extra morale boost.