The main thing I do is to try to strengthen their immune system. If they are raised by a broody hen the hen takes care of this. But for my brooder-raised chicks I feed them a small amount of dirt from the run where the adults are twice a week. This gets grit into their system, introduces them to any probiotics the adults have, and exposes them to anything their immune system needs to protect them against. If you do not have any adults, feeding them a little dirt still gives them grit and may introduce things in the soil they need to start their immune system protecting them. So expose them to their environment as soon as reasonable.
I try to keep the brooder, coop, and run dry. If they get wet and stay wet for a few days they can stink and harbor diseases. This includes keeping their drinking water clean. If the water gets poop in it and sits, after a few days dirty drinking water can harbor diseases.