A nutritionist I worked with for a couple of years advised me that ridges in the nails were related to inadequate protein intake. Most average sized women (I'm an 8) need around 20-25g with every meal. In general women eat lots of carbs, especially for breakfast, and almost no protein. I made a lot of changes based on his advice, and I haven't been able to keep them all up in this economy (good fish oil is expensive), but I get a decent amount of protein with every meal, and I have no more ridges. My nails grow like hard, hearty weeds. I like them short, so, hey, no help for me, but they look better. :)
I had the same problem. I apply finger nail polish to them,the "hard as nails" kind, clear or colored. It helps protect the nails so they want break & etc. It's worth the money, good luck.
Especially if your fingernails are not only thin, but are pitted and/or wavy you probably should have your thyroid level checked. That is what happened to my nails.