Your fingers are all muscles, and like all muscles you need to use them to keep them going. I have a rule where I'll do at least 15 minutes every day of something, and then go into focused practice. On the days where I'm just not feeling it, at least I did some maintenance on my fingers. I bought one of those "daily exercise for guitar" books. But you can really do anything: go up and down scales, practice a technique you see. Just don't keep playing once your fingers hurt. If you cut through your calluses you'll be unable to play for a few days if you're lucky.