See and play with both parents of the puppy before you buy. Puppies are cute. They soon become dogs. Make sure you want the dog he will soon become. His parents are the best indicator.
You cannot do this at a pet store. But that is OK, since there is ABSOLUTELY NO CHANCE that you will be shopping for a puppy at the pet store.
Keep in mind that my advice is mostly related to buying a purebred dog as a puppy. If you are adopting an older dog, the rules are completely different and my tips may or may not apply.