Contrary to popular belief, shaving your long-haired dog (or cat) will not keep him/her cooler during the hot, summer months.
Unlike humans, the skin of dogs and cats does not contain the vast network of blood vessels and sweat glands designed to dissipate body heat during hot weather conditions. True, dogs do possess sweat glands in their footpads, but these glands play a minimal role in overall thermoregulation. Despite being sweat-gland deficient, dogs and cats have an uncanny ability to vaporize large amounts of water from their lungs and airways, water that carries heat from the body when they pant.
Shaving pets for the summer can actually predispose them to sun burn and to heat exhaustion/heat stroke. Long hair and thick undercoats act as insulation against the sun's rays and their effects. Coats that are kept well-brushed and mat-free allow for good air circulation through the hair, which in itself can actually have a cooling effect. On the contrary, matted, unkempt hair coats stifle air circulation and do little to help cool the body. In other words, daily brushing is a must during the hot, summer months.
Here's a prime example: My 2 year old Boxer, Titan (who has a short hair coat) and my 8 year old mix, Gobi ( who has long hair with a thick undercoat) love to go jogging with me. Both dogs are extremely fit, yet after 40 minutes in the Texas heat, Titan's tongue is scraping the pavement, forcing regular water stops, whereas Gobi continues to just trot along like a canine version of Forrest Gump, seemingly oblivious to the heat. Keep pets cool and comfortable during the summer by keeping them well-groomed and by always providing a source of fresh water and shade. But don't shave them. If you do, you're only defeating the purpose and you may end up with a very expensive veterinary bill on your hands.