Don't buy from anyone advertising on the free ads/Gumtree/websites like Epupz, which is full of dodgy breeders - adopt from rescue or buy from a registered breeder recommended by the breed club.
Do your research, know what you're looking for and what to ask -including appreciating that a puppy farmer will often invite you to a family home as a "front" for their business and may even have another dog in that "normal family home" to further the pretence that they are a nothing more than a hobby breeder.
Never ever sell or give a dog to someone you don't personally know 100%. Preferably, if you have to rehome, do so via a reputable rescue.
Neuter or spay your own dog, dog theft is rife and stolen dogs are sometimes taken for the purpose of breeding/puppy farming. A neutered dog is less desirable. Likewise chip your dog and never leave him unattended, even in your own garden.
Join the campaigns of PuppyLove and read up on what they have to say.
Education is the key. Tell others what you've learned. Put it on your Facebook, put a link to PuppyLove as your signature on your emails. You'll be surprised at the number of people who come back and say, "Oh! I didn't know that!"
Personally I'd also say don't patronise PetPlan insurance - they knowingly insure puppy farmers, one of the main campaigners against puppy farming I know has a file as long as your arm on this which they've supplied to PetPlan yet still nothing has changed within the company.
Report anything suspicious to the local Trading Standards, to PuppyLove and come back here too as I'll pass it on to the other campaigner who I mentioned above.