Your feet and ankles swell when fluid in the blood escapes from capillaries and accumulates in tissues, a condition known as peripheral edema. Causes range from benign to serious and include being overweight, long periods of standing, lengthy car or airplane rides, warm weather, and menstrual periods.
Most remedies work by improving circulation and returning fluid to blood vessels. Get regular exercise (contracting leg muscles helps the veins pump blood back to the heart), and lose weight if you need to. When sitting or standing in one position for a long time, stretch your legs and move around every couple of hours. Support hose, which you can buy at drugstores, also promote good circulation. And consider cutting your salt intake to a teaspoon or less daily, which helps reduce water retention.