If your boyfriend is going out of town on business or for leisure, you don't have to spend this time moping. Sure, it's hard being away from each other for an extended period of time, but make the most of your time alone. Stay busy and lean on friends and family for support while you are apart. There are many ways to stay connected while he's away if he's in an area with Internet or cell phone reception.
Plan a time to talk daily or every other day. Use technology such as video chat if you can so you can see each other instead of just talking. If he's away on business, understand that this time needs to be flexible.
Keep yourself busy with activities you must do by yourself so you don't think about how much you miss him all the time. Buy a book to read while he's gone or knit that scarf you've been meaning to make for years. Now is the time to watch through your "Sex and the City" DVDs or your collection of romantic comedies he hates.
Go out with friends or catch up with friends you haven't spoken with for a while. You'll have extra time that would normally be spent with your boyfriend on dates, so fill this with other people. Go out for drinks or a movie, or stay in and make jewelry together.
Plan a trip during the same time with your family and friends. If you can't take a vacation as a couple, go on separate trips during the same time span.
Buy a body pillow if you miss lying with your boyfriend in bed. Cuddle up to the pillow instead of him and ditch it when he returns.