Like any muscle in your body, your feet should be exercised regularly to keep them strong and healthy. This can be as simple as walking a little every day, so if you don't get a lot of exercise in your day-to-day life already, try to make small changes such as taking the stairs instead of the elevator, or walk to the shops instead of driving.
There are also some simple exercises you can do with your feet, for example, stand barefoot with your feet flat on the floor and try to write the letters of the alphabet out with your feet. Another good one is to put a pen or something like a marble on the floor and try to pick this up with your toes. These exercises will help to strengthen the muscles in your feet.