Email every few months (now live couple hundred miles apart so no lunches), and as a PP said, I like to send articles that remind me of her and say so. SHe sends me that kind of correspondence as well. I've liked all my other supervisors but she was the best and I'd argue most connected.

I like to send articles that make me think of them. I think it's a low-awkwardness way to stay in touch and trigger a 'how are you' conversation.

Give a call every few months to catch up. A lunch every 6 months to a year depending on how good your relationship was. They also receive holiday cards and invitations to appropriate events (as in, if we have a kid of comparable age ill invite theirs to a bday party, or if im involved in a reception for my current employer all make sure my former is on the guest list).

I have former bosses/supervisors who I would like to keep in touch with on some level to keep my professional network circle active--you never know when you may need someone for a reference or something else down the line. How does one effectively do this?. They were my superiors so we weren't friends and I don't want to make anyone uncomfortable by shooting them "hey how are you" email every month but I also don't want them to forget me, or worse still call one for a referral out of the blue after several years of zero contact.

How do I keep myself on their radar but not come off as a pest?