[theGREENsuit]

Aside from giving people a call every so often, I use LinkedIn - messages, comment on their posts, etc. I also have a group of former colleagues whom I've worked with at multiple companies and we'll get together every few months. Depending on the size of your network, it may be nearly impossible to keep in touch with everyone on a frequent basis. In the past, I would invite a number of them to tech events around the city and that worked with some success.