I have a nice pair of over-the-ear, noise-cancelling headphones that I wear almost every day. The thing is, I wear them for so long they often develop their own damp, humid atmosphere. And sometimes they get a little smelly. Gross, I know, but I've found an easy solution.
I always save those little silica gel packets you'll find in packaging for electronics and food, and I'm always looking for ways to use them. To keep my headphones sounding funky and not smelling funky, I stick a couple of the packets in the ear cups when I'm not wearing them. Silica gel is a desiccant, or drying agent, that absorbs moisture. No moisture means no bacteria. They work great for your gym bag too.