When you dust a bird for lice or mites, you should redust 10 days later to kill the nits that have hatched since the first dusting. This will break their lifecycle. For severe infestations, a third dusting may be required. Keep in mind, at the same time you're dusting your birds...the inside of their coop needs to be dusted including nests...then when you go to redust your birds in 10 days, redust the inside of their house as well. Repeat as necessary. Here's a link with expert advice:
http://ohioline.osu.edu/vme-fact/0018.html
http://ohioline.osu.edu/vme-fact/0018.html