Sunscreen protects the skin from harmful ultraviolet radiation--UVA and UVB light--that ravage skin, damage its DNA, provoke cancer, and accelerate wrinkles. UVA has been dubbed the aging ray because, over time, the dermis damage it does produces wrinkles and pigment changes. UVB, the burning ray, triggers inflammation and dilates blood vessels--sunburn.

There's a lot of conflicting information out there about sunscreen and how you should use it. Here are my essential tips and guidelines for how to protect yourself from the sun's damaging rays this summer.

• Sunscreens that claim to provide "all-day protection" and "water resistance" are not reliable; ignore these promises and reapply all sunscreens at least every two to three hours and right after you've been sweating a lot or swimming.

• UV rays can pass through clothes, so invest in a few pieces of lightweight clothing specifically made with an ultraviolet protection factor, or UPF. A UPF of 50 means only one-fiftieth of the sun's UV rays pass through it. You can also use a laundry product with TinosorbFD to increase the UPF of your clothes; it'll last through repeated washings.

• Think one teaspoon, two shot glasses. Sunscreen only works if you use enough of it. Before heading outdoors, apply one teaspoon's worth of broad-spectrum sunscreen with SPF 30 or higher on the face and at least three ounces, two shot glasses full, on the body.

• You can still get burned on cloudy days--even when it's overcast. Eighty percent of UV light zips through the clouds.

• SPF 30 doesn't provide more protection than SPF 15--it provides protection for a longer amount of time. Anything over SPF 50 may give you a false sense of security. Regardless of the SPF number, reapply sunscreen at least every two to three hours and right after you've been sweating a lot or swimming.

• Apply sunscreen indoors, about thirty minutes before you go out in the sun. On beach days, apply sunscreen when you are naked to make sure you are totally covered before slipping into a bathing suit.

• Sunscreens expire--they have an expiration date for a reason. Most last for two years, so check yours and toss old ones.

• If a sunscreen makes your skin sting, itch, or break out, the likely culprits are PABA, avobenzone, benzophenone-3, helioplex, or octyl methoxycinnamate; switch to a formula without those ingredients. If you are sensitive to any of these chemicals, switch to any of the zinc oxide or titanium dioxide varieties.

• Always seek shade. It reduces UV by up to 95%. Sit under a tree or beach umbrella, walk on the shady side of the street, and park yourself on the protected side of a train, bus or car. Avoidance is your number-one tactic, especially between ten in the morning and four in the afternoon, and near reflective surfaces (sand, water, snow). Even when it's overcast, 80% of UV rays zips through the clouds.

• Don't forget to protect the lips, hands and ears. Those are the most common areas for skin cancer and most people forget about them.

Did you know that great skin isn't just the result of good genes? There are actually many factors, including lifestyle and stress that all add up to a glowing complexion. Dermatologist and guest editor Amy Wechsler has the scoop on how you can achieve the complexion of your dreams, no expensive treatments required. Board-certified in both dermatology and psychiatry, Dr. Wechsler treats the effects that state of mind and physical behaviors have on the skin. She is also the author of Mind-Beauty Connection: 9 Days to Reverse Stress Aging and Reveal More Youthful, Beautiful Skin. Follow her on Facebook and Twitter for more tips!