Many folk have so many different opinions of the muzzle. Of course anyone that sees a dog with a muzzle automatically assumes that the dog is dangerous. There are many types of muzzles but whatever one you choose make certain that the dog is able to breath.
The muzzle that I recommend is the Baskerville which is marketed by the eminent Roger Mugford. This is a basket design and allows near unrestricted mouth opening for panting and drinking. These are available at most good pet shops and many vets stock them. They come in 12 sizes to ensure that a good fit is available for most breeds of dogs. Available through The Company of Animals, PO Box 23 Chersey, Surrey KT16 OPU UK.
I encourage all owners to train their dogs to wear a muzzle, the earlier in the dog's life the better. Most young dogs are destructive, a muzzle will protect your home from being eaten. A muzzle will ensure that your dog is unable to eat any rubbish or poison on the ground, that alone is worth training for.
When visiting the vet it can also be useful for nervous dogs to wear a muzzle and also when a dog has been operated on. Wearing a muzzle is another way to prevent him from licking the wound. In the event of an emergency, if you do not have a muzzle then one can always tie the leash around the dog's mouth to prevent it biting.I would add that a muzzle does not cure a bad behaviour but it is useful whilst an effective therapeutic strategy is developed.David the Dogman