SIN You wear a baseball cap to protect your eyes
The shade of a hat's brim won't fully shield your eyes. Over time, exposure to the sun's UV rays speeds up the development of cataracts (it can damage the skin on your eyelids too). Wear sunglasses that block 100 percent of UV rays, like the Adidas Adistar ($150; adidas.com), under your hat.
SIN You don't want to tote a water bottle
Dehydration decreases your body's ability to sweat, which increases your risk of overheating. To prevent it, drink a glass of water before you head out, then fill up the easy-to-carry Asics grip bottle ($15; asics.com for stores), and bring it with you.
SIN You slather on sunscreen—but skip your lips
"They have less pigment than the rest of your face and are more susceptible to UV damage," says Amy Derick, M.D., a dermatologist in Barrington, Illinois. Apply balm with SPF every two hours.