Keep them filed down with a nail file or sand paper. If necessary shave or trim them first. If you're consistent with this, and don't suddenly change or increase exercises involving grip significantly, you should be able to avoid the vast majority of callus tears, and minimize the ones that do happen.
If you tear a callus, clean the area, trim it, and put vaseline on it.
While working out, you can try covering it with medical tape, although it's tricky to get it to stay on right. Just keep a roll in your gym bag so you're prepared. Plus it's also good for taping thumbs and scraped shins.