For a friendship to survive you need to do a bit of maintenance work on it, like a car. I suggest that you make an effort to spend some time together, even for just a quick coffee to catch up on news. Or you can txt/skype/msn to keep in contact. Always remember each other's birthdays.Sometimes you can email them a funny joke, just to say you were thinking of them. Some friends like to go to the movies, out for a meal or to a party/club.