Avon does a really great nail treatment at a bargain price - I think it's called Length and Strength Complex or something like that. Comes in a dark blue bottle and you just apply it once or twice a day to clean (ie unpolished) nails for a week, then whenever your nails need a bit of a boost between manis
I also really love Creative Nail Design's Solar Oil treatment, I apply it at night and then whack the Avon treatment afterwards before bed and my nails are super-strong Without using treatments they're brittle and split and flake if I so much as *look* at them the wrong way, so those two treatments work really, really well.

HTH