Do they free range or are they in a coop/run?
Keeping the run and coop dry and air circulating may help some.
Removing/managing waste and drying up damp areas.
Depending on your setup you can use fans to keep the air moving in the coop and run.
All of mine is covered, so I have a large outdoor fan (under cover) hung up that blows into the run, the chickens seem to like it especially since hot weather is here and flies/insects are kept at bay by the breeze. Also have a fan mounted in one of the windows of the coop (under cover).
These are just a couple of things I do to try to help keep pests down, but I'm sure others will have some great ideas.
