Keeping your dog well hydrated
One of the best ways to avoid dehydration is to always have sufficient shelter and fresh, clean water available for the dog. Heat stoke is a common cause of dogs becoming dehydrated. There are other causes of dehydration, such as severe vomiting or diarrhea or fever.
If you think your dog is dehydrated, you need to quickly determine exactly how bad the situation is. More serious dehydration shouldn't be handled by an amateur, so if the situation appears serious seek out a veterinarian immediately. IV fluids are a very important part of treatment in severely dehydrated dogs. In serious cases the situation can be even worse than normal dehydration because drinking enough to correct the issue can be problematic - dogs drinking too much water will frequently vomit, making the situation even worse.
Take it slow when you try to rehydrate a dog. If possible, use an electrolyte supplement rather than plain water. Avoid sport drinks since they have more sugar than a dog can handle properly, instead using a canine equivalent such as pedialyte. Give the dog small amounts of the fluid at very frequent intervals. If the dog is unable to drink on its own, use a medicine syringe and place the fluids in its mouth.