Let them know if/when you're looking to change job, or ask them for introductions if there is someone that could be useful to know for your work. And make sure to reciprocate by offering up advice or help or introductions when you think something might be of use to them.
You might be surprised what might pop up.
E.g. almost all my jobs have been referrals from connections or people who I've worked with, or even who have worked for me, that have later hired me another place, but in several cases they didn't realise I might be an option until I made it clear I was looking. Don't assume friends will seek you out for jobs etc. just because it's a fit - they may think you're happy where you are and don't want to be presumptuous, or it may simply not have occurred to them.
You might be surprised what might pop up.
E.g. almost all my jobs have been referrals from connections or people who I've worked with, or even who have worked for me, that have later hired me another place, but in several cases they didn't realise I might be an option until I made it clear I was looking. Don't assume friends will seek you out for jobs etc. just because it's a fit - they may think you're happy where you are and don't want to be presumptuous, or it may simply not have occurred to them.