Inappropriate discipline is a common encourager of aggression. If a friend or friendly person approaches you and your dog, and your dog growls or raise its hackles and shows his dental work, with your intent to calm him is touching him and speaking quietly, you just praised him for that behavior. The same goes for when someone is at your door and your dog jumps on that person as a greeting and you have the same reaction for calming, you just reinforced that behavior. Your dog just got the wrong message that this is the behavior which is expected from him, and if it goes uncorrected, the dog will eventually bite.
When you praise your dog, it will reinforce what action your dog just did. When a correction was needed by a choker chain or a touch to the side of the dog to redirect his attention. A correction is done properly when your dog stops what it is doing and turns its head and looks at you for direction.
Also, silence is a great way to communicated to your dog. More talk during correction is just feeding more energy into your dog to continue its bad behavior.
Proper corrections should only take 2 or 3 times for the dog to understand what you expect out of him. Remember, praise only when your dog is in a calm and submissive state of mind.
Here is the true source of the bond between dog and man: it is not who puts down the chow who wins the dogs heart but the one who handles him firmly, consistantly and affectionately.
Remember to stand strong on corrections or your dog will take you as a weak person in the pack and then come the issues that you are not happy with. A dog must trust and respect you in order to obey you.