Having come to this thread fairly late, I see tea tree oil has been mentioned already.
I agree, it is effective. I think it's best used with a nit comb. Rub tea tree oil into the scalp - all over, but paying particular attention to the danger areas, like behind the ears and the nape of the neck. Leave it for an hour or so, then comb through thoroughly.
An alternative that seems to work just as well is lavender oil, but other essential oils would be harmful to the scalp.
It would be interesting to know what sort of conditions the lice prefer. For instance, they may not like an acid environment, in which case, a rinse with lemon juice may help.
There are many web sites dedicated to the subject, and I think the answer is that there isn't an easy answer! You just have to persist.