Are you breeding purebred, show quality, layers, or pet birds? If purebred, make sure you start with the best quality possible.
If just for layers, meat, or pets then to make them as healthy as possible, try and allow them to free range, give a good quality commercial feed, you can investigate into Fermented feed if you dont do so already. Supplemental vitamins in the water, or Apple Cider Vinegar with the "mother" is also fantastic to add to the water. Make sure they have access to oyster shell and as clean of water as possible at all times. Clean dry bedding, and area to dust bath will also keep down mite/lice problems from arising.
If you want to take it a step further, you can add things such as PrioBios, seaweed, yogurt, garlic, sprouts, etc.....just depends on how far you want to take it.