D.T.
My sister became an "expert" when her daughter got it twice in one school year. Ugh! She bought all the OTC stuff, finally with frustration and no luck she called in the people from http://www.licehappens.com/
When my daughter got it last school year, my sister pointed me to the Nit Free Terminator comb!! It is amazing and the only thing you need! This special comb has teeth that trap the nits so they don't get brushed back through. We just combed her hair twice a day for a week and they were gone!
The people from LiceHappens had one that cost double. I found it on Amazon.com
http://www.amazon.com/Terminator-Professional-Stainless-T...
Good Luck!