If you skimp on sunscreen in favor of a shady umbrella or tree, you'll still burn. "The shade only blocks 20 percent of UV rays," says New York City dermatologist Jeannette Graf, MD. The solution: seek shade and wear SPF -- it's not either/or.

It sounds nutty, but the chances of getting burned through your white t-shirt are actually pretty high. "Your clothing is only SPF 4 to 9," says Graf. The amount of SPF depends on the color of the fabric: white gives you the least protection; dark and bright colors have a higher SPF because color absorbs more UV rays before they hit your skin. The tightness of the fabric's weave is also important. If you can see through your shirt when you hold it up to the light, apply sunscreen before you get dressed.

You're doing everything right -- you're religious about applying sunscreen, but you still get burned. The solution: apply sunscreen at night, says Graf. "If you always burn, and you're going on a beach vacation, start putting on sunscreen every night for a month," says Graf. "Either physical or chemical sunscreen is effective. The point is that application of sunscreen has a cumulative effect which enhances its effectiveness when sun exposure occurs."

There are two kinds of UV rays: UVA and UVB. B rays are the "burning" rays that fry your skin. A rays are the "aging" rays that cause wrinkles and sun spots. So even if your skin isn't tanned (or red), it's accumulating damage from UVA rays.

Here's the really interesting part: "UVA rays are the same all year round," says Graf. That means even when it's cloudy in January, UVA rays are wrinkling your skin. The moral of the story: wear sunscreen every day, all year long. Graf likes Glytone Sunscreen Lotion SPF 40, $32, which give broad spectrum protection with both chemical and physical SPF ingredients.

We hear the vitamin D excuse all of the time. A good friend insists she doesn't need SPF for our 45-minute runs because she's "getting her vitamin D for the day." Dr. Graf's advice: take a supplement.

"Vitamin D is a hormone, and it's important," says Graf. "But you can get it through food or take it as a supplement. And get tested to see if you're vitamin D deficient before you take action."