Sometimes the best maintenance is repair: reach out, even years later, and apologize for the thing that still nags at you.
The idea
Jayson Gaddis put it in a line that hit me like a ton of bricks: "The most important lesson you can learn in relationships is how to repair the conflicts within them." Conflict is normal and even necessary, whether it's an unanswered email or a long ghosting. Our default is to carry on as if nothing happened, or never re-engage. Coming back to heal it shows you care and have the integrity to own your mistakes. Sometimes the best way to maintain a relationship is to fix it.
How to do it
Ask yourself: is there someone I've had conflict with who pops into my head every so often?
Apologize to them, out of the blue, for the specific thing.
Example
Jayson made a room of high-achieving entrepreneurs cry by asking how often their parents apologized to them after a blow-up growing up. The same lesson applies to coworkers, colleagues and collaborators.
Script
"Hey, I know it's been a while, but I've been thinking about it a bunch, and I just want to apologize to you for ___."
Watch out for
This is for a real conflict. For plain silence, skip the apology and just reconnect (see Reconnecting After a Long Silence).
Who Do I Need to Forgive Today?: Add one journaling question, "Who or what do I need to forgive today?", to clear the grudges and guilt that quietly block outreach.
Who Have You Been Avoiding?: Pick the people you've been avoiding, send each a simple hello right now, and notice the dread was worse than the reply.
It's Never Been Too Long: Relationships have no expiration date. After any gap, assume they'll be happy to hear from you, and reach out with no agenda.
Relationship Debt: Some drift is a fair tradeoff if you take it on knowingly. Pay it back with a short, honest note instead of an elaborate apology.