Put sunscreen on 15 to 30 minutes before you head out into the sun. Most creams take more than 30 seconds to rub in. For lotions, use at least 2 to 3 tablespoons to cover your body. For sprays, apply as much as can be rubbed in, and then repeat. Reapply all sunscreen, regardless of its SPF, every 2 hours and again after you swim or sweat, since sunscreen can rub off or wash off during the day. (Some leave a residue on skin. Most sunscreens left stains that didn’t wash out of cotton, polyester, and rayon/spandex.). Limit your time in the sun, and if possible, wear protective clothing, a hat, and sunglasses. Check the expiration date. The FDA requires manufacturers to provide an expiration date or show that a product will remain stable (but not necessarily maintain its SPF) for at least three years. If you buy an undated sunscreen, mark the purchase date on the container and, to be prudent, toss it once it’s two years old. Don’t store sunscreen in a hot car—it may degrade faster. And skiers take note: Once frozen, sunscreens may lose effectiveness.