If you bite your nails into shape, you'll leave behind small tears in the edge that will gradually lengthen and widen. It probably goes without saying, but DO NOt bite your nails, even for the purpose of shaping them.

And avoid the almost irresistable temptation to use your fingernails to tighten a screw "just this once". I can't tell you how many times I've had to re-learn that lesson.