I'm amazed no-one had pointed out diet yet - Calcium is a necessary nutrient for strong fingernails and you need plenty of vitamin D to help your body absorb it. Also your nails are made up of layers of keratin — a protein that's also found in your hair and skin. Plenty of protein is also good for maintaining healthy strong nails, I'm sure there must be vitamin suppliments available that promotes healthy nails.
Also avoiding soaking your hands for long periods of time ie: washing up or having a bath that can soften up and weaken your nails.
You can try a nail hardener, but avoid products containing toluene sulfonamide or formaldehyde. These chemicals can cause redness or irritate the skin, and thats not a good idea for guitar players is it now.
-Pet