What causes shoe odor? That is a simple enough question to answer. If you have smelly feet, you will have shoe odor. Foot odor is caused by bacteria that breed between the toes when your feet are moist.
To prevent smelly feet, it is important to soak and dry your feet often. If the foot odor is strong, then soaking and drying your feet should be done more than once a day. In some instances, as often as four times a day is necessary. After the feet have been soaked and dried thoroughly, powder them and put on a thick pair of white dry socks. Do not put the dry clean feet into a pair of smelly shoes. Keeping your feet clean and fresh smelling is one way of preventing shoe odor.
One way to prevent shoe odor is to heavily coat the shoes with baking soda. Baking soda is a natural deodorizer. If your feet regularly have a heavy odor you do not want the shoes to absorb that odor; therefore, after you wear the shoes sprinkle a heavy coating of baking soda into them. Place the shoes in a plastic bag and put them into the freezer overnight; this will likely kill any bacteria that may be in the shoes from the smelly feet. If you do not want to put the shoes into the freezer, then spray the inside of your shoes with rubbing alcohol. After this, spray them with baking soda; this will kill any bacteria that may have accumulated from your foot odor.
Keep in mind that shoe odor is caused by bacteria; in order to prevent shoe odor you have to prevent an accumulation of bacteria in the shoes. The way to do that is to be sure that your feet are always clean and are not moist. That moisture is what causes the bacteria to build up on the feet and causes foot odor which, without proper care, then transfers to the shoes.
In summary: to prevent shoe odor you have to prevent putting smelly feet into your shoes. Wash your feet often and dry them thoroughly, especially between the toes where bacteria breed. Put the clean, well-dried feet into a thick pair of white cotton socks. Do not wear synthetic fabrics, as they do not absorb moisture as well as cotton socks do. Then place your fresh, dry feet into your shoes, confident in the knowledge that you've prevented any odor which may accumulate in your shoes.