I was working with a client tonight who said he really liked using the clicker - his dog really responded well to it. The more we talked, the clearer it became that he was using the clicker as an attention-getting-device, not as a tool to communicate to the dog that he was doing something good, something that would earn him a reward. He was clicking when his dog was barking. He said the dog quit barking as soon as he heard the click and ran to him for a treat. In the short view, this sounds great, but...
You get more of the behavior you click for. So if the dog is barking when you click, you're teaching your dog to bark MORE!
Clicker training is very powerful stuff, but only when the human's timing is right. An easy way to remember when to click: you get more of what you're clicking. So be careful what you click for, you'll definitely get more of it!