The things below were learned via a lot of heartbreak. It's long, but I found these to be very important for their long-term health and well-being:
#1. Do a daily check for "fluffy bums". If their hindquarters are soiled, they're likely sick. Prompt treatment saves lives. I did this each morning before anything else by taking a handful of scratch, waiting until they're clustered around my feet (because they know what I'm about to do [see #2 below]), and then toss it as far away as I can. They all turn and run, showing you their behinds. This can also reveal if any are injured or ill in some other way.
#2. Routine. It keeps them calm(er) because they know what to expect, and it helps ensure you cover everything that they need on a daily basis. My morning routine was to:
check their bums;
clean, disinfect, and fill their waterers (I kept diluted dishwashing soap, a gentle scrubber - luffa works great- and a spray bottle filled with a very diluted bleach/water mix to spray before then rinsing and filling.);
fill (and clean if necessary) their feeders;
top off oyster shell (don't feed crushed eggshells as they dissolve quickly, decreasing their urge to take the longer lasting oyster shell) and grit;
remove most of the nightly poop under the roost;
fluff up the bedding (I used straw) and toss some of it under the roost, adding more straw every so often, as they pulverize it; and
check nest boxes for eggs and replace bedding if soiled (we dried grass clippings and used those as it was soft and easy to pick up just the dirty patches).
#3 Keep on top of parasites by using broad spectrum "wormers" and rotating among wormers to prevent resistant pests and treat different spectrums of parasite. Some parasites can last a long time outside the body and are difficult to kill, leading to constant reinfection, so preventing them in the first place is important. Parasites can affect how well your chickens absorb nutrients and that in turn can cause issues like thin shells that break and cause peritonitis, a death sentence. Beware of "alternative" worming suggestions. They may have merit, and you can do them in addition, but do you want to use your chickens as experimental labs given the health consequences of parasites? I did sprinkle diatomaceous earth under their roosts in the summer as it kills fly maggots.
#4 Check frequently for bumblefoot. Clear the run of small sharp rocks, nails, glass, etc. Check feet at least once/week. It's so much easier to treat at that stage. Just pull or scrape away the scab (tweezers work well) and clean out the pus, put antibiotic ointment on it, put the pad a band aid over it, and wrap the foot (not tightly) with strips of vet wrap (around the ankle once, then crosswise over the foot, gently through the toes on one side of the foot, back over crosswise and gently through the toes on the other side, repeat a few times, then straight across the pad, then back up and around the ankle and press in place.) Repeat every other day until healed.