There are many external factors that can affect your skin, but two stand out as the most profound: sun exposure and smoking.
Sunlight emits three types of radiation: UVA, UVB and UVC rays. The UVC rays are the most dangerous, but our planet's ozone layer does a good job of blocking them. We Earthlings do need to worry about UVA and UVB, both of which play a role in premature aging and skin cancer. To prevent these conditions, be sure to use a sunscreen with a sun protection factor (SPF) of 15 or higher that offers protection from both UVA and UVB rays [source: Skin Cancer Foundation]. And keep in mind that the SPF of a sunscreen does not refer to an amount of sun protection. Rather, it's an indication of how long after applying the sunscreen it will take to burn your skin. For example, using an SPF of 15, it will take 15 times longer for the sun to burn your skin than if no sunscreen were applied. Ultimately, it's just best to minimize your exposure to sunlight. Seek shade during the hottest part of each day, wear protective clothing and hats whenever possible, and use sunscreen daily. And keep in mind that older skin is more vulnerable to the effects of sun than younger skin, so an SPF of at least 30 should be used.
Smoking accelerates the aging process and causes skin to wrinkle prematurely [source: Mayo Clinic]. And the effects are not limited to the face -- smoking is associated with skin damage all over the body. The main cause is nicotine in tobacco, which restricts blood flow to the skin, causing it to be oxygen starved and more prone to pruning. But smoking increases the risk of wrinkles in other ways as well, such as through the effects of chronic heat exposure to the face and the repeated flexing of certain facial muscles used to inhale and exhale smoke. The result is often a discolored and pinched effect, also known as "smoker's face." So if you want to keep your skin healthy for as long as possible, don't smoke.
In addition to avoiding sun and smoke, there are a number of other practical lifestyle modifications you can make to keep your skin in top form, most of which are firmly rooted in common sense. For example, you should protect your skin from extreme heat and cold, avoid using harsh soaps and detergents, moisturize regularly, and keep alcohol consumption to a minimum. It's also important to minimize stress, get optimal sleep on a regular basis, and maintain a healthy weight. In other words, take good care of your whole body and, chances are, you will be rewarded with healthy skin for life.