Normal, healthy children get lice; they needn't have poor hygiene. Be sure that head lice are really what you are dealing with, because it's not always easy to make the diagnosis. Young girls tend to get lice most often, but they are also the group that is most often misdiagnosed. Other insects, dandruff, scabs on the scalp, or plugs in hair follicles are often mistaken for lice.
Treatment generally includes the application of permethrin cream rinse, which can be bought over the counter. Apply it to the hair, leave it on for 10 minutes, then rinse it out. Because it does not kill all of the eggs in a single application, it should be used again in 7-10 days. Then use a fine comb on the hair to remove the nits. Clothing and sheets should be laundered in hot water or dry cleaned, and hairbrushes should be replaced or soaked in permethrin and then boiled. All household contacts should also be treated for lice at the same time.
Assuming you have done all of these things, most treatment failure is due to repeat infections from household contacts, clothing, or hair-care items. A stronger permethrin solution or a lindane shampoo may be suggested. If the condition doesn't improve, have your child evaluated by her doctor.