This article is from the Training Your Dog FAQ, posted to rec.pets.dogs newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
You should never correct when you yourself are upset, angry or downright mad, especially at your dog. Good correction depends on timing, a keen awareness of what the dog is thinking, and quick switching between correction and praise, all of which are difficult when you are upset. Stop the exercise until you regain your equilibrium. You will have much difficulty training your dog if you continually get mad while doing it. In fact, if you always or often get mad when training your dog, someone else should train him. You will get absolutely nowhere yelling at your dog.
The dictum "don't train before 6 months of age" doesn't make any sense unless you're talking about the _correction_ involved in formal obedience training. If you think about it, you train your dog all the time whether you realize it or not. Dogs are great at picking up your body language and tone of voice. Even if you're not trying to train them, they're "training" themselves using the clues we give them (and many "problems" are classic cases of the dogs misunderstanding their owner's signals).
If possible with a young puppy it is best to use the "correction" of distraction. When you deny the puppy something, try to replace it with a positive activity rather than just being negative and oppressive all the time. Otherwise, limit your corrections to a verbal "no."
Most dogs at some point will refuse to do something that he knows how to do. this is independent of how he has been trained. Striking out for independence appears to be a semi-universal mammalian trait, judging from the behavior of human adolescents. However, you must be prepared to enforce the idea that the dog does not really have an option about doing what you tell him to do. Otherwise the dog will increasingly choose whether or not to obey you and become unreliable. You do have to know the dog you are training and be able to tell the difference between confusion and refusal. Correcting a confused dog is quite detrimental. Learning how to tell the difference is part of being a trainer. While no one can really teach you this skill, you do have to learn it.
Always praise the dog immediately when he listens to your corrections. Again, this gives the "jekyll and hyde" feel to dealing with your dog. But it is very important to immediately praise your dog for listening to you. This helps build confidence and keeps the dogs from having that "hang-dog" look when performing.