I've used the Sally Hansen fingernail treatment in the past. I found it peeled off the nails too easily, so I gave it up.
My Beautiful Wife recently (6 months ago?) twisted my arm to try Nailique nail hardener. I'm much more satisfied with it. It does seem to help. You paint it on the nail(s) once a day, and you need to remove it for a day once a week or so.
If my picking fingernail(s) is broken so I can't use it, I'll use a plastic fingerpick, reversed. I used a nail clipper to reshape the tip, and then soften it in hot water and straighten out the "hook" of the pick a little. It's louder than playing with fingernails, but the sound is fairly similar. I wouldn't use metal picks because I couldn't shape them in either way mentioned, and the sound is wrong for my taste too.