It is much easier to teach a dog to do something good than it is to teach it not to do something bad. When breaking a dog of an undesirable behavior, the first thing to consider is what reward the dog is getting for it. He must be getting some reward or he wouldn't keep doing it, but sometimes rewards are subtle or counterintuitive. When a dog jumps up on his owner and the owner pushes him off, being handled is the physical reward and may be more powerful for the dog than the punishment of yelling. Handlers must be careful not to accidentally reward dogs for undesirable behavior. Once the handler identifies and eliminates the accidental reward (to the extent possible), the next step is often to train an incompatible behavior. The dog trained to sit when the owner picks up the leash cannot simultaneously jump up and knock her owner over.
The clicker is a training tool and handlers should not use it indefinitely. The purpose of the clicker is to communicate the desired behavior. Once the dog understands the command and performs it reliably, you can eliminate the clicker. You can still reward the dog, but over time you may shift from a highly prized food reward to a less desirable food reward, then perhaps to just a verbal reward. When the behavior is learned completely, no reward may be necessary at all, though dogs as well as people appreciate feedback for a job well done.
Operant conditioning is a powerful tool in shaping the behavior of almost any animal, including dogs. A focus on positive reinforcement helps everyone enjoy training and deepens the bond between the trainer and learner. The use of a marker helps accurately pinpoint the desired behavior and greatly speeds the training process. When a dog fails to learn something, it's often because of a breakdown in communication rather than an unwillingness to cooperate. A good trainer can help troubleshoot these problems. The only real limits of this kind of training are the ability of the handler to identify meaningful reinforcers correctly, and to break the desired behavior into manageable steps.