Make sure that your shoes fit properly. Purchase new shoes later in the day when feet tend to be at their largest and replace worn out shoes as soon as possible. At the beach or when wearing sandals always use sunblock on your feet as the rest of your body. Some people's feet sweat more than others, and are more prone to athlete's foot. Wear shoes made of leather or canvas, not synthetics.
Sandals are good.
Switch shoes from day to day. Use foot powder. See your doctor if severe problems persist. Check your feet every day. Wash your feet every day.