I have also been there with bells on, mine were getting reinfected every time I cleared them, you can usually tell as there will be one or two fully grown huge nits if the infestation is recently caught from another child, whereas if there's lots of smaller ones, it's probably that you didn't clear the eggs when you treated them last time and they've just grown big enough to see them.
My tip is to buy the bug-busting kit from this charity
www.chc.org/
It's about £6 or free to those who have free prescriptions (you can ask the doctor to give you it). They are just brilliant on this website, they explain the whole life-cycle, and how you can break it.
So, if I get an outbreak now (which is rare), I shove on tonnes of conditioner, sit them in front of the TV with a lolly and comb through the hair, using the increasingly finer combs. I've found other combs not to be fine enough and to let the eggs escape; they have a purple one that gets the tiniest eggs. You do this every two to three days (they say how often, I can't actually remember), I find two or three go's is enough and then you are free. You then keep on top of any further outbreaks by checking the hair regularly so as to catch any adult marauders from other children's heads quickly before they lay many eggs, just when you are washing or combing in the morning.
Two other things help this; regular trims so their hair isn't knotty otherwise the combing is really a pain with long hair and I second the person who said put it in plaits too.
Good luck with it, I don't work for the charity (honest), but I think their comb pack is brilliant and has also been tested and the results published in peer review journals (yes, there are academics who do this!) so it's not just some company out to make money of you repeating not that effective or potentially toxic treatments. I found Hedrin rubbish, which is why I have just resigned myself to the odd 10 day comb-out if required.
I love the idea of saying loudly about nits to the offending parent!