Most dogs are less likely to get sunburns than humans. A dog's coat acts as natural insulation from damaging ultraviolet light. However, dogs are susceptible to the sun in area that are not covered by fur- the nose, the tips of the ears, and the belly.
Dogs with fair skin, short hair, or white coats are also more susceptible to the sun than other dogs. The ultraviolet rays are able to penetrate the coat and attack the dog's skin directly.
Try to take hikes in the woods in the middle of the day and save the beach for the early morning or later in the day. Besides the sun being less brutal the beaches as less crowded and the dogs have more room to play.
Another option is to cover up the sensitive area of the dog's body with a t-shirt or a hat. In a pinch a shirt does help cover a good portion of a dog's body. There are also a number of canine apparel products available.
There are two key rules to follow if you use sunscreen on your pet:
1) The sunscreen should have a SPF of 15 or higher to be effective. Although the higher SPF formulas offer more protection they tend to be thicker and more difficult to work with on your dog.
2) The longer the sunscreen is on your dog the more beneficial it will be. Get a water sports sunscreen or something that is designed to stay on in the water especially if you are taking your dog swimming.
If you dog gets sunburn try spraying with cool water. A bath of cool water and baking soda will also help relieve the burn and the itch. For extended relieve you can use a lotion version of any of the popular sunburn relieve products.