How can I stay in touch with former colleagues? – Ask a manager

A reader asks:

I’ve been working with a senior colleague for a while now who has been great: knowledgeable, supportive and patient. He recently left my organization. When he left, he told me to keep in touch.

I’d like to keep in touch, but I have no idea what to say! I feel like the standard advice is to send relevant articles and say why I think they would be interesting to him, but he has moved to a different industry and is over 10 years older than me. Should I just reach out and ask him how the move went and what the new job is like? How do I follow up afterwards? What are the best ways to stay in touch with colleagues in general if you no longer work together?

I’m answering this question today at Inc., where I’m revisiting letters from years ago that are buried in the archives here (and sometimes updating/expanding my answers to them). You can read it here.