As soon as possible. In addition to stopping further UV exposure, dermatologists recommend treating a sunburn with:  

• Cool baths to reduce the heat.
• Moisturizer to help ease the discomfort caused by dryness. As soon as you get out of the bathtub, gently pat yourself dry, but leave a little water on your skin. Then apply a moisturizer to trap the water in your skin.
• Hydrocortisone cream that you can buy without a prescription to help ease discomfort.
• Aspirin or ibuprofen. This can help reduce the swelling, redness, and discomfort.
• Drinking extra water. A sunburn draws fluid to the skin surface and away from the rest of the body. Drinking extra water prevents dehydration.
• Do not treat with "-caine" products (such as benzocaine).

If your skin blisters, you have a second-degree sunburn. Dermatologists recommend that you:

• Allow the blisters to heal untouched. Blisters form to help your skin heal and protect you from infection.
• If the blisters cover a large area, such as your entire back, or you have chills, a headache, or a fever, seek immediate medical care.

With any sunburn, you should avoid the sun while your skin heals. Be sure to cover the sunburn every time before you head outdoors.

For more information on sunscreen and general skin care, visit the American Academy of Dermatology website.   This information can be found in greater detail there.