This article is from the Classical Guitar FAQ, by Joshua Weage (jpweage@mtu.edu) with numerous contributions by others.
One policy that may help is to rub skincream or nail cream
into the cuticles of the nail, twice a day. This will strengthen the
nails in a general way and make them more resistant to damage.
The following tips have also been successful for at least the author:
1. Avoid getting your nails wet. Use rubber gloves when you
have to wash dishes, the car, etc.
2. Get into the habit of using your left hand for things such
as opening doors/drawers, flicking switches, anything where
you might catch the nails on something.
3. Let your left thumbnail grow a little long so you can use it
for prying instead of the right one.
4. Keep the nail edge very smooth with a file and ultra fine
sand paper (around 600 grit). This will prevent little snags
which can catch on things and maybe rip off the nail tip.
5. Apply two or three layers of clear nail polish. Put polish
on the overhanging underside of the nail too.
6. Don't let the nails get too long. They can hamper your playing
and they may break easier.
 
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