The best way to prevent skin cancer is to avoid over-exposure to the sun. Cover up and use a high factor sunscreen of SPF 15 or above (children and those with fair skin will need to use a sunscreen with a higher protection factor) and avoid going out in the sun between the hours of 11am and 3pm.
This is not to say that you should avoid sunlight all together. Sunlight promotes the production of Vitamin D, which is needed by the body to produce calcium. Also, recent research has shown that Vitamin D can prevent other types of cancer, and cardiovascular diseases (conditions that affect the blood flow), such as strokes.
When spending time in the sun, you should do so in moderation. You do not need to get a suntan to enjoy the benefits of Vitamin D.