Just regularly calling (or e-mailing or texting) your friends every so often, especially if it’s been awhile since you’ve heard from a specific friend, would be a nice gesture. If it comes from out of the blue from you, then it does show them that you are proactively being a friend. And most of all, that you care enough to do the things friends do for each other. Like checking in with them. If you keep this up, you will have another problem…too many friends to keep in touch with!