Every so often I set out to "make the rounds" and see my friends who are spread across the country. I'm a bit of a nomad and yet its often my return that triggers close friends to see each other when they live minutes away. Just pick up the phone and call someone from time to time, let them know you give a shit. It's amazing what a phone call once a month and a visit once a year can do to sustain a meaningful relationship with someone.
People are, on average, terrible at staying in touch with one another. Be the outlier, you'll make the world a better place.