When you take off your shoes, do you notice a terrible odor? Don’t be worried, it happens to many people. After all, your feet sweat, and are tightly encased in that shoe for many hours at a time. It’s only natural that they get a bit stinky from time to time. The question is what you are going to do about that scent.
You can spray different deodorizers in there, if you have them on hand. Or, if needed, you can simply pour a bit of baking soda into a coffee filter, close it up, then leave it in your shoe overnight. In the morning, you can toss the baking soda filter out and be confident in knowing that your shoes no longer smell.
via Lifehacker
I personnaly spray a mixture of formol and demineralized water (20%-80%) into my shoes an I wait till they have dryed before wearing them again. Also prevents mycoses.