Best way to dry boots?

So I finally got rid of my old smelly boots, and I don't want the same fate again. With my old boots, I had just been air drying them/putting them on a vent and letting them dry that way, but after two years of use they smell like something is dead in there. My shoes don't smell (not even the ones I've had for four years and still wear regularly), so I assume it's some sort of issue with them not getting dry?