If you are considering a show dog, remember that no one, not even an expert, can predict with one hundred percent accuracy what a puppy will be like when he grows up.  The puppy may seem to exhibit show “potential” but realistically four to six months is the earliest age for the would-be exhibitor to select a show prospect and know that its future is in the show ring.

NEVER BUY ON A WHIM!!!!!!

DON'T BUY FROM A PET SHOP OR PUPPY MILL.

Telephone or email the breeders and ask about the breed. Most will be happy to answer any questions you might have and will advise you of the suitability of their breed to your situation.
Decide if you want a pet or a show quality pup. This may determine how long you have to wait and how much you will have to pay - show or breeding quality pups usually being fewer in number and more expensive.
Have a look at the health of the other dogs in the kennel. Are they lively alert and friendly? (Don't forget most kenneled dogs will bark at strangers and some breeds will very rarely look friendly but you can get a general idea - dogs that look down right viscous might not be good parents for a pet dog).
Does the kennel look clean and sanitary? Do the dogs react happily to the owner/handler or do they cringe, snap or growl?
Puppies should be nicely rounded in shape without being overly fat or skin and bones.
Most healthy pups will have shiny alert eyes and healthy looking coats.
Check that the puppies are not infested with ticks and fleas. Ask to see both parents. It is common for breeders to use a stud dog that they do not own but they may have photos of the dog. Quite often the bitch will not look in 'show condition' after whelping a litter. The reason for this is that feeding a litter places huge demands on the bitch so she may be a little thin (but not skin and bones) and may 'blow' her coat (shed)due to the hormonal changes caused by pregnancy and milk production. She should however still appear happy lively confident interact well with the owner/breeder and show no signs of illness.
Ask the breeder if they have any references from people who have bought their puppies in the past. Make sure that you see all appropriate paperwork including parent’s pedigrees medical certificates for tests such as hip dysplasia if this is relevant to the breed as well as the puppy’s pedigree papers and vet certificate confirming worming and vaccinations. Be very skeptical of any breeder who is not willing to show you paperwork - if you are unable to see the papers the breeder may not actually have them! Many people are not concerned about having 'papers' for their pet however you do want to make sure that if you are paying the price for a pure bred dog then that is what you are getting. Also if the breed that you are buying are prone to certain medical conditions you want to make sure that the parents have been 'cleared' by a vet and the puppies checked (depending on the relevant condition)to ensure that there is the smallest chance possible that the pup you buy will develop the condition.
 

What else should I look for in a healthy puppy?

Nose:
Nose should be cool and moist and not running. If there is a nasal discharge or frequent sneezing this may be a sign of a respiratory tract infection.

Mouth:
Check the teeth and gums. Most breeds have a scissor bite which is when the upper incisors tightly overlap the lower. Gums should be pink and healthy looking. Pale gums indicate anemia. The puppy should have clean smelling breath.

Throat:
Check the back of the throat, enlarged tonsils can mean tonsillitis.

Ears and Eyes:

Eyes should look straight ahead and not deviate to the side. If tear staining is present on the muzzle look for eyelids that are rolled in or out, extra eyelashes or conjunctivitis. White spots on the surface of the eye could be scars from prior injuries or infections. Ears should stand correctly for the breed. Tips should be healthy and well furred. Crusty tips with bare spots suggest sarcoptic mange. Ear canal should be clean, wax free and sweet smelling. A buildup of wax or a rancid odor may be caused by ear mites. Head shaking and tenderness about the ear indicates an infection of the ear canal.

Chest:
Feel the chest to see if the heart seems especially vibrant. Puppies should breathe in and out without effort. A flat chest, when accompanied by trouble breathing in may indicate an airway obstruction.

Coat and Skin:
Skin and hair around the anus should be clean and healthy looking. Signs of irritation such as redness or hair loss indicate the possibility of worms, chronic diarrhea or digestive disorder. Coat should be bright and shiny. Excess scale or itching in the coat suggests mites, fleas or other parasites.

Structure:
Legs should be straight and well formed. Structural faults include legs that are bowed in or out. Gait should be free and smooth. A limp or faltering gait may be the result of a sprain or hurt pad or hip dysplasia or other joint condition.

REMEMBER:
It is ultimately your responsibility to research the breeds you are interested in as well as the individual breeder that you finally purchase from and while it is your right to ask as many questions as possible it is also the breeders right (and responsibility) to ask you questions as well so don't be offended if the breeder gives you the third degree!