Most dermatologists would agree: In a perfect world, everyone would wear sunscreen every day, clear skies or cloudy. Slathering it on is, after all, your best protection against skin cancer and premature aging -- aside from not going out in the sun at all (and what fun is that?). The thing is, whether you're a 365-day-a-year sunscreen addict or more likely to apply it only when basking for that well-baked bronze, the lotion won't do you any good unless you choose it and use it right.

READ THE FINE PRINT. Not all sunscreens are created equal. So before you buy, check the bottle for one of three ingredients: Parsol 1789 (also called avobenzone), titanium dioxide or zinc oxide. Although they work differently -- Parsol 1789 absorbs ultraviolet (UV) light and the other two reflect it -- the end result is the same: broad- or full-spectrum protection (the label will undoubtedly tout this attribute). Broad-spectrum sunscreens block both UVA rays, which are linked with a higher incidence of melanoma and cause premature aging (wrinkles, brown spots, freckles), and UVB rays, which cause sunburn.

Your drugstore is probably stocked with numerous acceptable options -- Coppertone Shade Sunblock Lotion SPF 45 ($9.50) or Neutrogena Active Breathable Sunblock SPF 30 ($9.50) are two -- but the best way to choose is by trial and error, says Richard Castiello, a dermatologist with a private practice in Chevy Chase. "Find something that isn't too expensive that feels good on your skin," he says. If you're prone to breakouts, you can try an oil-free version, such as Clinique City Block Sheer Oil-Free Daily Face Protector SPF 15 ($14.50).

GET SPF SAVVY. Sun Protection Factor indicates the amount of anti-UVB power in a product. The higher the SPF, the longer you can stay out in the sun without burning. But how much is enough? Castiello says that an SPF of at least 15 is usually sufficient: Less than that, and the level of protection plummets dramatically; higher than that, and the difference in protection is smaller, though 30 is still slightly better than 15, 45 is better than 30, and so on. He adds that "dark skin has a higher threshold to burning because it contains more melanin, which is the body's natural defense against sunburn," so a lower SPF is plenty for darker-skinned people.

Tina Alster, director of the Washington Institute of Dermatologic Laser Surgery, has one piece of advice when it comes to SPF: "The higher, the better."

APPLY LIBERALLY -- AND OFTEN. "You can't overuse sunscreen," says Alster, who recommends using one ounce (the equivalent of a full shot glass) to cover all exposed areas of the body. She stresses, though, that one application is not enough if you're outside all day. "Reapply every two hours or after swimming or heavy perspiring," she says. And don't worry too much about labels that say "apply 30 minutes before exposure to the sun"; popular belief may be that the skin needs time to absorb the lotion, but Alster and Castiello say it's not necessary -- it's more important to just get that sunscreen on.

It's also important not to be fooled by labels that say "waterproof." Because they're usually stickier, waterproof sunscreens "do have a bit more staying power," Alster says. "But they will eventually rub or wash off, especially if you're really active." Bottom line: Reapply, reapply, reapply.

Jennifer James

Another good thing about sunscreen: You can always ask someone to help you rub it in.