Lesley Miller might not have known until it was too late. Five years ago a friend spotted a dark mole on her right shoulder and urged her to get it checked. "The dermatologist took one look and said, 'That has to come off'," remembers Miller, a fair-skinned south Florida resident who'd grown up at the beach. "It was terrifying."

A biopsy confirmed the mole was melanoma, the deadliest form of skin cancer. Miller, then just 31, had it removed in a procedure that required several stitches and left a scar. The cancer hasn't recurred, but she's had 12 precancerous growths taken off and she scrutinizes her skin for suspicious moles. "People think skin cancer is something that affects people in their 50s or later," says her dermatologist Dr. Alysa Herman. "But that's not the case anymore."

White men older than 50 still comprise the majority of people diagnosed with melanoma. But it's now the second most-common cancer among women in their 20s (behind thyroid). The rates of less serious skin cancers have also risen sharply: the incidence of the most common form--basal cell carcinoma--tripled in women under 40 between 1976 and 2003 (to 32 cases per 100,000), and the rate for squamous cell carcinoma quadrupled (but to just four cases per 100,000), according to a 2005 study. Dermatologists expect the numbers to increase with the enduring perception, especially among young women, that a tan is attractive--a message reinforced by sun-worshiping celebrities like Paris Hilton. Experts say the proliferation of tanning beds, which emit ultraviolet rays, hasn't helped, either. Certainly, exposure to ultraviolet light isn't the only cause of skin cancer--heredity plays a role, too--but it's the main one that can be avoided.

The good news: if treated early, the cure rate is high--95 percent or more for basal cell and squamous cell carcinomas. Even with melanoma, the five-year survival rate is 98 percent when tumors haven't spread. And remember, it's never too late to start protecting your skin.

Know your skin: If you burn and don't easily tan, be especially careful. (Those with dark skin, like African-Americans, are less susceptible, but not immune.)

Protect yourself: Use a sunscreen that blocks both UVB and UVA rays and has a high SPF (sun-protection factor). Lotion with 15 SPF blocks about 96 percent of the sun's rays, but a 45 SPF cuts out about 99 percent. Apply about a shot glass of sunblock and reapply every two hours.

Get screened: The American Cancer Society recommends screenings by dermatologists every three years for people 20 to 39, and annually after that. But those with particularly fair skin or a family history may need to be screened earlier and more often.

Be aware: Look for any changes in your skin--particularly asymmetrical moles or spots that change color or size. If a growth or skin lesion doesn't heal within a few weeks, see a doctor.

Know your options: The standard melanoma treatment is excision. In advanced cases, chemotherapy or radiation may be used.

Precancerous growths, and some basal cell or squamous cell carcinomas, can be frozen or scraped off. But if you have several growths, a doctor may prescribe chemotherapeutic cream, or try laser or photodynamic therapies, which can target several lesions in one session. An increasingly popular treatment for nonmelanoma skin cancer on the face or neck is Mohs surgery, in which layers of the skin are cut and examined under a microscope until all cancerous tissue has been removed.

Skin cancer is the most common form of the disease, but also one of the most preventable. "I had no idea what kind of damage the sun was doing to me [as a teen]," says Miller, who has since changed baby oil for sunblock. "Back then, we just weren't educated on it." That's an excuse no one can use anymore.