The main way to prevent your flock from getting bird flu (avian influenza) or any disease is biosecurity (a fancy term for doing everything possible to keep disease away from your own flock and to keep from spreading disease to your neighbor's chickens). In terms of a poultry flock this means keeping your birds away from other birds. That includes other people's flocks as well as wild birds.
Wild birds or any new birds you bring into the flock can carry diseases that could spread quickly to your flock. Having a bird feeder out to feed the local wild birds, for example, is not a good idea. Wild birds are an important source of avian influenza.
Here are some things you can do:
If you have taken your birds to a fair or poultry show, keep them separate from the rest of the flock for at least two weeks after the event. The same thing should be done if you've bought or were given new birds. While it doesn't guarantee that your birds won't get sick, it will help to reduce the possibility.
Webinar: Biosecurity and backyard flocks
Dr. Jacquie Jacob
University of Kentucky