Breastfeeding to age 2, at minimum.
Babywearing. Keeps baby away from germs his immune system isn't ready for yet.
Avoiding daycare, and other school-type settings in the early years. We are lucky enough to homeschool as a lifestyle, though we do attend weekly church meetings, with my 18 month and older children in classes.
We wash hands (or try to!) before eating, after coming home from the park or store. I have hand gel we use in the van when we can't get to a sink.
In the flu season, (fall, winter, and in early spring), we take more precautions: less sweets, make an immune boosting soup with herbs once a week, drink more water and herbal teas. I have tinctures and homeopathic medicine on hand to give when cold and flu symptoms start. Not crazy stuff, just more conscientious. Oh, and for fevers, for very stubborn high fevers, I have given Motrin to my kids and it didn't impair their bodies ability to heal, as I had been scared into believing when my oldest was a baby. I don't give it until their temp is past 102/3 and I feel like it's necessary.
We eat relatively healthy; I am the one in the household who is buying and finding ways to have whole foods in our family's diet. My DH is the levity, who buys ice cream
No alcohol, tobacco, etc.
I like "Smart Medicine for a Healthier Child" as a reference. It helped me establish what I needed for a first aid and home health kit and how to tell when my kids had a certain illness and how to treat at home, as well as when medical care was indicated. Invaluable reference as I could avoid taking them in unnecessarily to the pediatrician. Aviva Jill Romm's book is excellent as well. (Naturally Healthy Babies and Children and her Vaccine book)
My oldest had unexplainable seizures at age 3. really glad he was unvaxed so I didn't have to worry if that played a role! He never has had them again. All three boys have had pertussis, full-blown, and we all survived. I am SO glad we are immune now! No more worries for that illness.
I keep in mind being healthy isn't about NOT getting sick, but about finding ways to support one's body to be able to resist disease, and if one gets sick, to heal from it sooner. Illnesses, when not excessive, tends to build the immune system, giving it a chance to build back up and become stronger.