Obviously protection from exposure to the sun is increasingly a widespread concern with the rise of skin cancer and other disorders. Because of Tibet's high altitude, the sunlight is far more intense and potentially damaging than lower altitude areas around the world. Thus, even if you are not usually worried about the sun, whether simple burning or more long term adverse effects, you should take seriously the need to protect yourself. Many people get their first serious sunburn in Tibet. In addition to burning, the sun can be extremely hot on your head to the point it causes headaches, while its general affect can lead to fatigue, sunstroke, and other problems. Consider yourself warned!!
One of the easiest ways to protect yourself is to avoid the sun. Never purposively lay out in the sun just to get tanned - you will find yourself picking up a tan regardless. This is especially true from 10am to 4pm when the sun is strongest. In general, when you have the option stand in the shade, or walk down the shaded side of the street. In hot weather, wear light clothes that covers as much as of your body as possible, such as long sleeved shirts. Given the drastic and rapid temperature changes, as well as the great temperature difference between shade/sun and day/night in Tibet, such clothing will be more consistently comfortable anyways.
Most importantly, wear a hat! Even if you are not in general someone who wears hats, a hat is an absolute necessity in Tibet both to protect your face from burning and to protect yourself from headaches, sunstroke, and other problems. Given how hot it can get during the day in Tibet, and the need to have the hat with you to easily take off and on as the situation requires, you may want to invest in a hat which can be easily folded up and packed away. Look for a broad-brimmed hat on all side to provide maximum protection to your face and neck. In addition, consider a high tech hat which offers sun protection via special fabrics (indicated by the product description specifying a SPF rating of 30, 50 or whatever - SPF is the number indicating how much something protects you from harmful aspects of sunrays).
Sunglasses are also essential equipment not only to protect your eyes, but also simply to enable better vision during the day.
Everyone needs to apply suncreen to their face, neck and any other unclothed parts of the part every day, regardless of wearing a hat or anything else. Despite the importance of sun screen, however, it is also important to pay attention to the above guidelines as well since sunscreen alone will not protect you. You should use about two tablespoons of sunscreen each time you apply it, apply it about thirty minutes before you go outside, and reapply it every two hours.
Every sunscreen (also called "sunblock") indicates its SPF (sun protection factor) level - the higher, the more protection. SPF 15 protects against 93 percent of UV rays, SPF 30 protects against 97 percent, and SPF 50 wards off 98 percent. However, SPF only measures the level of protection against UVB rays, not UVa rays. There are ingredients in some sunscreens that can also block or absorb UVA rays, but you have to check to see if they are available in any given suncreen. They include: avobenzone (Parsol 1789), octocrylene, titanium dioxide, zinc oxide, and Mexoryl SX. You can also check to see if the sunscreen packaging says it is "broad spectrum," which is supposed to mean that they protect against UVB and UVA rays, or it may even explicitly refer to the issue. A separate issue is whether the sunscreen is comfortable to wear, which mostly relates to how "greasy" it feels. Cheaper sunscreens feel very greasy, often resulting in people wiping or washing it off during the day because if feels uncomfortable. Other suncreens are hardly noticeable upon application, and are thus much more comfortable to use regularly.
One excellent, though expensive, sunscreen is "Neurtrogena Age Shield Sunblock" (recommendation - June 2008). It has a SPF of 45, blocks UVA also, and is remarkably non-greasy in application.
One important thing to keep in mind is that sunscreens must be reapplied every two hours. The typical active ingredients begin to break down when exposed to the sun." There are exceptions such as zinc oxide which stay potent after two hours, but not all sunscreens are made with these ingredients.
As for exposure to water, don't count on sunscreen to last through hours of swimming or showers. or sweating, regardless of what the label may say. The term "water/sweat/perspiration resistant" indicates a sunscreen offers SPF protection after 40 minutes of exposure to water, while "very water/sweat/perspiration resistant" means it still protects after 80 minutes.To be safe, reapply sunscreen after swimming or sweating.