I just eat lots of salads and fruit! I also take a multivitamin every morning.
My nails grow very quickly (my hair too), and I actually have to cut them every 2 weeks, otherwise they get too long. They also never split or break off. I only paint them very occasionaly, so that also helps. I've heard that nail polish makes them brittle in the long run, and it's true because my nails have never looked so good as when I stopped painting them all the time.
I also extend my hand cream to my nails and cuticles.
nail Growth 
Linear Mode
