Doctors say sunscreen is one of the best ways to prevent skin cancer. "Skin cancer is an epidemic in the U.S.," says Roger Ceilley, MD, a past president of the American Academy of Dermatology and clinical professor at the University of Iowa School of Medicine. "Somebody dies of melanoma every hour, yet most of these deaths can be prevented with hats and sunscreen," he says. From wbmd

The SPF number gives good protection information about the UVB radiation which is the radiation which will get you to burned. However it does nothing for telling you about the UVA radiation protection which will cause wrinkles and signs of aging later in life. It is the UVA radiation that is the leading cause of skin cancer. Unfortunately there currently is not a way to rate UVA radiation. Thus trying to figure out what products give the best UVA protection is very difficult. In order for a product to have much of any UVA protection it must contain oxybenzone, octylmethyl cinnamate and cinoxate, sulisobenzone, salicylates, titanium dioxide, zinc oxide, or avobenzone (Parsol 1789). and the label will say "broad-spectrum" on it some where. Reapply often products with avobenzone in them as it breaks down in sunlight fairly quickly. Products with zinc oxide do not have to be that gaudy white stuff any more. Get one with the micro particles of zinc oxide. For UVB protection, i.e.. sunburn protection, get a product with at least SPF 15 Sunscreens need to be applied 20 to 30 minutes before they begin to work properly.

SPF 2 => 50% of UVB rays reflected
SPF 15 => 93% of UVB rays reflected
SPF 30 => 97% of UVB rays reflected.
Products with SPF Greater than 15 do not buy you a lot more protection.

It takes 15 minutes or more after application for sunscreens become effective. Thus you need to put them on before you go out into the sun. Most people do not apply enough sunscreen. Lather that stuff on baby and rub it not your skin well. Sun reflecting off water can increase your exposure to suns's radiation. Re

Don’t forget that lips get sunburned too, so apply a lip balm that contains sunscreen, preferably with an SPF of 15 or higher.

Sunscreen should be applied liberally to form a film when first applied. Sun screens should be applied 20-30 minutes before hand as it takes that long to be absorbed into the skin. Otherwise You can get burned before the sunscreen becomes activated. Apply sunscreen last after other lotions. Re-apply after sweating a lot or getting out of the water. For sure after a couple of hours. Water resistant formulations may be good for only about 40 minutes the experts say. Sunscreen chemistry breaks down after about three years so replace your 5 year old tube now. Sunscreens typically break down from excess heat so leaving the bottle in the sun or in a hot car can affect it shelf life. Thus replace your bottle more frequently than you think you should. The UV rays penetrate the clouds so you need to apply sunscreen on cloudy days.

For mild sunburns:
use cool clothes. Take frequent cold showers. Apply a lotion containing Aloe Vera. Steroid Creams such as 1% hydrocortisone cream can help. sunburns can promote dehydration so drink fluids to avoid headaches.

Preventing wrinkles with sunscreens
Facts about Sunscreens - American Academy of Dermatology
Sunblocks - Skin Care Rx website
Sun Screen Consumer Report - Consumer Search.com