I use a fine grit emory board, like you might use to round off your nails. it might sound like it would ruin your callouses, but if you use an emory board just enough to keep the callouses from getting to the point where they're catching your strings and causing problems, and actually use lotion, coconut butter or shea butter, on the tips to keep them from drying out... they'll stay hard from you playing, but 'supple' enough, and round enough, so they won't tear. I've heard nothing but good stuff from other mandolin and guitar players, as well as hand drummers, doing this to keep callouses 'healthy' so they can play. There's nothing quite as bad as when a dry callous at the tip of your finger actually cracks open if you don't take care of them... takes awhile to heal and playing can be super painful. I keep a little emory board, as well as some toughskin glue (like bowler's use) in my case so when it happens, I can take care of it! Glueing cracked callouses sucks and burns, but can get you through a performance!
Good luck!
