Replace any old goggles with a new pair that contains an anti-fog coating. Make sure the goggles fit properly to prevent air and water from getting in, both of which can contribute to fog. The seal on the goggles should sit comfortably on the bridge of your nose and around your eyes, firmly pressing into your skin to prevent gaps. Adjust the strap that wraps around your head to be secure; the goggles should stay securely in place while swimming.
Repeated use of the goggles can cause the anti-fog coating to wear off. If that is the case, apply anti-fog spray or drops on the lenses of the goggles, following all manufacturer directions; methods can vary slightly by product. Alternatively, place one small drop of dish soap on each lens, followed by a rinse in cool water for one to two minutes; using more than the recommended amount of soap can be damaging. Avoid rubbing the lenses, which can cause the fog to worsen.