Most people don't call a dog trainer until they already have a problem. Sometimes they call as soon as they realize they have a problem or the beginning of a problem. More often they call after the dog has been engaging in the behavior for far too long. Usually something unpleasant or even disastrous precipitates the call. Generally, I find that for those dogs, it is too late. NOT because the dog can't change - it's the humans that can't (or won't) change! If they've waited so long to "fix" a behavior, they usually don't have the persistence and patience to adhere to a behavior modification program. Then, I'm caught between a rock and a hard place. If I don't take the case they are upset with me. If I do take the case and they don't adhere to the program, the program fails and they blame me! What's a girl to do?
Best I can do is try to convince everyone I know to start training early and stick to it. Yes, it is a lot of work over the first year of your dog's life, not to mention the ongoing maintenance - let's not forget that. Good training is a lifelong process. Oh, but it is well worth it! You may have your dog for about 15 years - think of how much unpleasantness you will go through in the course of those 15 years if you don't work to train your dog! My monsters are a constant source of pleasure to me because I spent the time to make it so! And, believe me, they are not little robots. I have trained them to be good pets (ok, so they do a few stupid pet tricks too), not to compete in obedience trials, although Bam Bam probably could. That's all I need and all I want and we are all happy.
I also tell people that you don't always have to do "formal" training sessions with your dog. The most successful training programs are ones where the owners have built the training into their everyday life. Life with dogs can and should be wonderful and rewarding, not stressful and annoying. But, as with anything worth having in life, it takes some effort!