Clean,clean clean...... Along with lots of TLC and love.
1) Clean Water
We use small containers for water and electrolytes. That way we rinse out and fill several times a day so they always have clean water and electrolytes to drink.
2) We use dry hay from our field and clean coop every other day.
3) Lots of TLC. We pick up and inspect the flock one by one at least three times a day. This I believe is the most important part. They get used to being handled, so if they do get sick it is easier to get medicine, and care for them. You can catch illness, mites and bugs quicker because you know your birds and how they look when they are healthy. You know your birds and can tell when they are not feeling right.
3) Clean pools. We keep kiddy pools up even for Chickens and Turkeys. They will cool themselves on hot days by standing in the nice cool water. After morning swim time with the ducks if water is dirty we will clean them out and refill.
4) Good Food. We give our flock a very good variety of food. We give feed in small amounts, for the same reason as water. You can always give more. This way containers stay clean... And so does feed. We allow free range during the day. When you are making veggies or fruit for the humans cut up leftovers or peels for your flock.
5) Have an alternative coop to isolate quickly sick family members. Also have separate feed and water containers for your isolation coop.
6) Keep medicine handy at all times. Time can sometimes really matter when family members are sick. Always keep the electrolytes on hand.
Remember the first and foremost. Prevention is better than losing your loved ones.