Diet is the best way to strengthen the nails (certain foods, nutrients, etc.). Nail strengtheners that are applied to the nails directly that use natural oils and nutrients to strengthen the nail work well but avoid types with formaldehyde. They not only can turn the nails yellow (not a problem for people that wear nail polish), but they also make the nails less flexible. So while they get really hard, they also get quite brittle and will break rather than flex if put under enough force. Clear nail polish or polish type strengthener (I've tried Sally Hansen Thicken up and Nail Tek II and both are decent) with the shine buffed out work okay for a temporary fix while trying to use dietary methods to strengthen the nails. Be careful though as some nail polish is harsher on the nails than others and nail polish remover unless acetone free can really dry them out.
One Ring to rule them all, One Ring to find them, One Ring to bring them all, and in the darkness bind them.