Before buying, look for a breed specific rescue. There is one for almost any breed. That way you can get the breed of dog you are looking for and save a life. They can be found by googling the name of the breed plus the word rescue or searching at: http://www.akc.org/breeds/rescue.cfm. The adoption process at many of these organizations can be laborious but it is worth it and it is the right thing to do. If you feel you MUST buy a dog (and there is no real reason you must) only select breeders listed on the AKC website, and then be sure to look for licensing and animal welfare violations at http://www.aphis.usda.gov/animal_welfare/
Visit the breeders home, meet the mother, make sure she is well cared for and a part of the family – reputable breeders do not keep their dogs outside! Ask to see veterinary records for both parents. Make sure all the puppies look healthy and if you get a bad feeling about anyone or anything – do not proceed. And last but definitely not least, reputable breeders do not advertise sell their dogs at pet stores, 100% of those dogs come from puppy mills where the breeding dogs are treated deplorably and the puppies are often very sick.