Sunscreen should be applied a half hour before going outside, even on cloudy days. It should be reapplied every couple of hours and immediately after swimming or sweating. Even if you buy water-resistant sunscreen, apply it generously to every inch of exposed skin and reapply it as often as regular sunscreen. Remember that sunscreen is not a license to stay longer in the sun. Wear sun-protective clothing, such as long sleeves, and stay in the shade whenever possible.