Before you start anything at all you may want to do a search of this forum or Google and figure out just what you have - - lice, mites, etc. Some pests reside on the birds, some in the coop, and they each have preferences on where they lay eggs, etc. Knowing the life cycle helps, too, so the first step is ID'ing your pest.
If you already have a severe infestation you may have to up the ante. Poultry dust would be my personal choice; it contains permethrin, is relatively safe for animals (but not insects), and is easily found at the local co-op or
TSC. I use DE in my coop shavings, and poultry dust on the girls.
Some folks dust the whole bird, but I have had a good preventive maintenance results with dusting their bottoms and under each wing every 1-2 weeks. I would be interested to hear what large farm or professional poultry show people do.
It's a crummy job, but you may also want to strip your coop of all shavings/hay, scrub it with diluted bleach when the birds are not present, let it dry out, and treat the floors with a layer of poultry dust or spray before covering with shavings. Some pests lay eggs in wood crevices of the coop.
I do know it is very important to nip this problem right away; mites and lice can kill your birds.
Good luck!
Jen in TN