Training a dog is all about reinforcement. What behaviors are you encouraging, and how are you encouraging them? Sometimes you don't even know you're reinforcing a bad behavior, and this is something a keen trainer can help point out to you.
The most obvious reinforcer is food and treats. And they work. Most of the time. Combined with a clicker to point out the precise behavior you approve, treats can be powerful and effective.
But what if your dog is trying to do something you don't want it to do, and the reinforcer of food just isn't getting through? He sees a squirrel, and the promise of "playing" with that squirrel has consumed his mind, and nothing, not even food, could be better than even the mere expectation of playing with that squirrel. And so he pulls on the leash. Constantly.
Problems like these can be solved with a little creative thinking.
We ran into this when we brought our new daughter home. All Bauer wanted to do was lick and smell and look at the new baby. He would get in the way, try to jump, attempt to get on the couch - all sorts of bad behaviors we had tried to make extinct.
He ingored our typical training methods. Food and treats, no matter how much he was starving, were pointless. He didn't care.
So what was the perfect reinforcer for this situation? The baby herself. I would stand with the baby and Bauer would begin going crazy again. Not until he sat still for 15 seconds or more did I click, and then bend down slightly to let him smell the baby. Pretty soon he was sitting and staying still more often than not when we had the baby.
And since the baby was a very, very strong reinforcer, this happened more quickly than other behaviors and situations.
Now that the newness has worn off a bit, Bauer doesn't care as much about the baby as he used to, and so the baby can't really be used as a good reinforcer. It's just a normal member of the family (which means he still gets excited, but not stupid excited.) So things change. And a good owner needs to be aware when they need to make those changes in reinforcement.
But even if the reinforcer might not be a reinforcer forever, taking advantage of it at the right time when you have the chance helps solidify more of the behavior you want in the long run.