There are lots of people who mix the DE in with whatever their chickens take dustbaths in. Just make sure it is the food grade kind and you don't need to use a ton to where the dust will be bad for them to breathe.
OOPS! I forgot to mention that DE definitely helps to control lice and mites.
When I change the hay in my coop, i sprinkle the nests, and the new hay really good with DE, I also spread a good handful on the roosts, Hope that will keep them from getting any mites or anything else.