I usually use a light gauge string set and have no problems with breaking nails from playing. Heavier strings usually clip the top edge of my nail but working around the house and especially working on engines does my nails in every time. I tried many different things as suggested above. Gelatin DOES work but it takes a while to realize benefit. I once used a plastic finger pick, turned "backwards" and carefully filed and sanded to conform to a normal finernail's shape and feel. Worked just great until it flew off, landing.... somwhere. I began to play without fingernails when ever one broke and after making a tape playing that way I realized that it sounded very much like Clarence Ashley (not the playing, the sound of the fingerless nail hitting the string!). Maybe he had fingernails breaking too!