It's amazing how the myths of head lice continue.
The lice that infest your clothes, those are body lice, not head lice...
Head lice cannot live more than 24 hours off the human body, and only transfer from person to person directly. There is a theory that the reason why kids catch them more because they tend to touch heads with other kids...head lice can't physically jump, but they are very fast climbers.
Most pharmacy treatments will tell you to treat, wait one week and treat again.
I'm not a fan of using 'tea tree' as a preventative, because I worked with tea tree oil for 2.5 years in a lab situation (developing pharmaceuticals with tea tree oil) and out of our team of 8, two developed contact allergies to the oil, so I would not consider it a good idea to use it 'every day' - it's quite toxic.
Generally when they re-occur it's the same infestation (you didn't manage to eliminate all the eggs the first time) - the lice are easy to kill, the eggs are very difficult to kill, so that's why you wait a week - until the eggs hatch, then you knock them all off before they have time to reproduce.
There is a 'dimethicone' based product that we used here in Switzerland - I found it to be very effective - it's called "Hedrin" - it's non toxic, doesn't sting in the eyes, and isn't a pesticide based product, and you put it in the hair, leave it overnight - wash it out in the morning - it works by suffocating the bugs - and it really does seem to work...
Please don't do anything too drastic. They aren't dangerous or poisonous, only about 20% of people are actually allergic and will itch, the other 80% of kids/adults have them and don't even know that they are there.
Head lice like straight, clean, not-too-oily hair. They are much harder to see on dark-coloured hair, and they tend to stick around the backs of the ears and across the neckline, where the skin is warmer.