Wash your feet every day with antibacterial soap and water, and make sure you dry them completely. Wash your feet in the tub if you're not taking a shower.
Give your shoes a break -- never wear your shoes two days in a row. Letting your shoes dry out completely between wearings kills bacteria. And hey, it's an excuse to go buy some more shoes!
Always wear socks with shoes. But you don't have to sacrifice style -- "invisible socks" or "no-show socks," which are designed to hide under ballet flats or loafers, will do just fine. Wear cotton socks or tights, if possible, since cotton allows your feet to breathe. Wear tights or pantyhose if you wear sandals.
Go barefoot as much as you can to allow your feet to dry out. Sleep without socks, go barefoot around your house, or secretly slide your feet out of your shoes while sitting at your desk at work -- whatever it takes!
Wear shoes that fit properly. If your shoes are too tight, your feet may sweat more than usual (and you'll be miserably uncomfortable, too). Even better, choose shoes made of natural fabrics that breathe-- then you can breathe easily, knowing your feet are in good shape.