I second the Apple Cider Vinegar mix for use in prevention of thrush. I creates a pH that's hostile to the nasties that cause thrush, yet doesn't harm the horse. It's also non-toxic to humans, which is a huge plus, and it's cost effective.
You can actually use ACV to kill the thrush as well,just mix it a little stronger and apply daily for a week or until it clears up. This mix is better for mild infections. If you have a stubborn case, there are better treatments.
Generally, just trying to let the feet get some air everyday does a lot for prevention. It's been sooo wet in parts of the country, it's harder than usual to keep thrush at bay. If you can provide a dry spot for your horse to "loaf around" that helps. At the very least, picking the feet out every day will help, even if they go right back in the mud. Any exposure to air is beneficial for the frog.