It's a common myth - limit your sun exposure so that you slowly get tan over time, without burning, and you haven't caused any skin damage and can safely tan.
A search on the internet will turn up a number of articles that tell you have to "tan safely." But remember, just because someone wrote it, doesn't mean it is true. Most medical experts will tell you there is no such thing as a "safe tan."
The Myth of the "Base Tan"
The confusion may come because it is true that having a "base tan" prevents you from getting sunburned. Therefore, it is thought, this base tan helps protect you from developing skin cancer. The problem with this thought process is that it is not only sunburn that increases your chances of developing skin cancer but any damage to your skin increases your risk.
Dr. Francesca Fusco, assistant clinical professor of dermatology at Mt. Sinai School of Medicine says, "One of the most common myths I come across in patients is that a base tan will protect against burning and subsequent damage... Damage to your skin from the sun can happen in just minutes from exposure to ultraviolet rays." 
A suntan indicates that you have damaged your skin. Research has recently shown that when your skin becomes tan, you have actually caused DNA damage and that excessive sun exposure, whether over a long period of time or shorter, more intensive exposure will both cause skin damage and can lead to skin cancer later in life. 
What Happens When You Tan
Your skin reacts to the sun's (or tanning bed's) UV rays by producing more melanin. And while we see this as desirable and "looking healthy" it really isn't. Sharon Miller, a scientist at the U.S. Food and Drug Administration explains it this way, "A tan is the skin's reaction to exposure to UV rays. Recognizing exposure to the rays as an 'insult,' the skin acts in self-defense by producing more melanin, a pigment that darkens the skin. Over time, this damage will lead to prematurely aged skin and, in some cases, skin cancer." 
Getting Enough Vitamin D
Sunlight does help our body's synthesize vitamin D and that is important, but many doctors agree that only 15 minutes in the sun is enough to give our bodies as much of this vitamin as we need. And according to the Skin Cancer Foundation, getting vitamin D from a combination of diet and supplements is a safer way to make sure you have enough.
If you crave that sun-kissed look, the best bet is to use self-tanning products. Even a small amount of sun exposure each day quickly adds up. And each time you go out in the sun unprotected, you not only increase your chances of skin cancer, you move a little closer to prematurely aging your skin.
 "Any Tan Is a Sign of Skin Damage," 2009, Dec. 1, Staff Writer, UPI.com
 "Is There Such a Thing as a Healthy Tan?" 2009, June 3, Staff Writer, The New York Times
 "Ultraviolet Radiation: A Hazard to Children and Adolescents," 2011, Feb 28, American Academy of Pediatrics Policy Statement