This is probably the easiest way to avoid sick birds. Over-cleaning is not necessary. The birds need to have access to droppings to maintain natural "gut flora". I know it sounds awful, but it's absolutely necessary for their health.
Excessive dust in their coop. It can cause respiratory problems for them, and sneezing/allergy fits for you. I hose the coops down entirely on a regular basis (3 months or so) to keep them as dust-free as possible.
Wet Spots in the bedding are a source for "bad" bacteria to breed. Chickens don't seem to know when to steer away from bad food, etc.. when they are scratching around. The fungus, spores, mold, and bacteria that breed in wet spots can cause respiratory and gut problems for your birds. If water spills, clean up the wet bedding, and allow the area to dry. Constantly wet areas in their environment should be managed. Holes in their yard that stay wet, and filled with mud, leaves and the like should be cleaned up and filled in.
Adding water to dry food is never necessary, and just creates another environment for "bad" bacteria to breed.
KEEP FEEDERS AND WATERERS CLEAN!!! In the summer, various algae and organisms will breed in your chickens' waterers at a phenomenal rate. None of these are good for them. They cause chronic diarrhea which saps strength, dehydrates the birds, slows growth, and lowers their disease resistance. This is probably the single most important aspect to keeping birds, especially in the summer, when their water intake is so much greater. Wet moldy feed will do the same to your birds' health.
Use Quality Feed. Commercial feed is wholly nutritious. Supplement with greens, and an occasional snack of cracked corn, of scratch grains for darker egg yolks.
In times of stress an Electrolyte/Vitamin supplement can help. At the onset of cold weather, or in extremely hot temperatures, it's a good idea.
Shade is important when the temperatures soar. When planning your coop, try to provide partial to full shade. Whether by trees, or by covering their yard with shade-cloth, it will benefit your birds. It will also keep their colors vibrant.
Dust Bathing is essential to you birds. They need to roll, and bathe in "dust holes" that they will dig for themselves. It's fun to watch them. The dirt helps control mites, lice, and "stickies" on their feathers and skin. When it is not available, a pan filled with sand or dirt placed in the coop will do the trick. Additives such as Sevin dust can be used cautiously if mites and lice become a problem. During long winter months, this is sometimes necessary. Bug overload will drive them nuts, and cause picking, and stress.
Cover from the elements is critical. They need to be able to get out of the rain, and weather. They can handle all kinds of temperature extremes, but cold and wet and windy is a situation no bird can tolerate for long.
Watch your birds as often as possible. If they show a loss of their normal "vibrance" something is wrong. Try to isolate the problem, and make corrections as soon as possible. If you spend plenty of time enjoying them, they will "tell you" if something is wrong.
If your birds do get sick isolate the sick bird/s from the healthy ones, and try to find out what the problem is. Pay attention to everything they are doing. Pick them up and examine them, try to notice anything that doesn't seem right. Watch them. Get in touch at the message boards, and email anyone you think may help. Take them to the vet if you like. The main thing is, work FAST!!!
Medications are available for most all problems that your birds may encounter. Prevention is always cheaper, and easier. It is very important to fight disease with the proper medications when necessary. As with human medications, the best, most up current medications are VERY expensive.
It is essential to properly diagnose the problem that your bird is suffering. With poultry many maladies will cause similar symptoms. If you are medicating for one problem, and it is actually another, the medicine will most likely be ineffective.
It hurts to buy the most expensive medicine available, but usually you spend just as much trying to fight the sickness with two or three other medications, only to find that they didn't work.
As with every other lesson I've ever learned, I learned this one straight through my wallet.
There are many websites devoted to helping you analyze symptoms in your birds. I admit to not knowing much about doctoring my birds, because I haven't had to. Sick birds are your best teachers. If they do get sick on you, you will soon know a lot more about chicken diseases than you ever wanted to know.
Read up on how to keep your birds healthy through Biosecurity. It'll save you a world of grief!!!