Find a sunscreen formula that fits your life and lifestyle," suggests Sandra Read, M.D., clinical instructor of dermatology at the Georgetown University School of Medicine in Washington, D.C. "If it's too heavy, sticky, or expensive, you're not going to wear it."
Scan the label for "active ingredients." This will tell you which ones are actually doing the protecting. Good choices include the highly effective sunblocks titanium dioxide and zinc oxide, and the chemical sunscreen avobenzone (a.k.a. Parsol 1789).
Look for "broad-spectrum" on the bottle, too (sometimes written as "complete" or "total protection"). That way, you'll know you're guarded against UVB and UVA rays. Here's why this matters: Sunlight emits up to 95 percent UVA (the rays that penetrate deeply into skin, causing wrinkles, spots, and skin cancer) and around 5 percent UVB (the more energetic rays that cause sunburn and can also contribute to skin cancer). "You may still be exposed to massive doses of UVA if your sunscreen's not broad-spectrum," says James M. Spencer, M.D., associate professor of clinical dermatology at Mount Sinai School of Medicine in New York City.
Go a little higher on the SPF. "Sun protection factor" is a lab measure of the time it takes skin to sunburn when you shine UVB light on it. With an SPF of 15, it should theoretically take your skin 15 times longer. But since most people apply less than half the amount of sunscreen used in lab tests, they get only about half the protection. That's why most dermatologists suggest using an SPF 30.
Don't be lulled into a false sense of security by those really high SPFs. "They don't offer that much more protection," says Ava Shamban, M.D., a dermatologist in Santa Monica, CA. An SPF of 30 absorbs about 96 percent of UVB rays, but a 45 is only about one percentage point better. Still, you should use them if you have very fair skin or have already had skin cancer. "Your sunscreen will be stickier, and you may get slightly better protection," Dr. Shamban says. Try: Coppertone NutraShield SPF 70 ($10.49, drugstores) and Cetaphil UVA/UVB Defense SPF 50 ($14, drugstores).