Eat. Gelatin.
Find out the foods that contain natural gelatin, such as fish and foul skins, etc. and also plain old "Jello" - and add it to your weekly diet regimen. No need to overdo this one, but not paying attention to it can be the kiss of death for the guitarist's nails.
Check out Tommy Emmanuel and the use of Thumb Pick with fingernails. Also check out how he grabs the thumb pick much like it would be a standard plectrum when circle picking those awesomely fast and accurate single note lines.
For Flamenco, which I understand Joe is studying right now, I'd go with Paco de Lucia method. Nylon strings and fingernails.
--Mac
Tommy does not use his nails. His nails on his right hand are cut as short as possible. He has callouses on his right-hand fingers and that is where he gets his tone. I had the chance to talk with him about this about a year ago. He said he never could get a good sound with his nails but found that if he built up the callouses it was just what he was looking for.