I try to keep in the habit of calling my circle of friends at least once a month. You really have to reach out to friends because everyone gets busy and its very easy to lose touch and bam a year or two has passed.
I'm careful of what types of activities I get involved with. I have friends who'd rather throw back a few drinks at a bar and call that recreation. I'll minimize my time at bars and clubs because its really a waste of time.
Social, religious and charity groups are usually good calls for finding decent like minded people.