Tea tree oil in the shampoo helps to prevent them. Some say it kills them as well. Cooking oil left on the hair overnight is supposed to smother the eggs and loosen them.
I did this on my 9 foster kids (group home) before they could go back to school. They have a nit (lice eggs) free policy. It takes several washings to get all the oil out but they have soft hair afterwards and oily hair is much better than lice.