I use baking soda and peroxide for a lot of things, even whitening teeth. I wrote a hub on it. It's a natural whitener. Also, don't forget the white pencil! My mother used this back in the day, amazing how people forget to use them now. Get them in any makeup section. That's for under the nail. Also vinegar keeps them clean. I usually do that just before I change my nail polish.
Tip: If you wear black nail polish, they will tint, again, peroxide and baking soda. I have a toothbrush I use just for that. Use like 2 times a day, should get whiter in a couple of days.