Which approaches are effective and why?
Most therapists begin their practices with what they learned in graduate school. Because of the current biases and behind-the-scenes power struggles of the profession, therapy methods are currently required to be empirically developed and shown to be empirically effective, which generally winnows itself down to Cognitive Behavioral Therapy (CBT). In my opinion, this is a therapy that is minimally effective, but perhaps, as it is technique based, works more rapidly for those who need to see immediate and specific change. It is narrow and does not dig very deep. However, lasting change is another issue and cannot be accomplished in ten sessions any more than you could have been raised as a child in ten hours.
Lasting change involves more sophisticated and long-term intervention. It is based most strongly in the relationship with the therapist. The psychology literature is littered with more studies than I can count demonstrating that the kind of therapy a potential therapist tells you that she practices is not as important as the relationship the two of you can develop. The stronger the influence, the more she will be able to help you make the changes you want. Of course, she should have a toolkit of different ways to get there.
Two aspects that must be learned by every therapist and deeply developed are an understanding of important influence of context and the complexity of the human psyche. That is, she must understand the context of your issues. She must take the time necessary to get to know you. Secondly, she must be aware of the complexity of the human psyche and environment. Nothing is ever caused by one thing and often cannot be resolved by one simple and quick change.
I do not mean to imply that you need to spend years on the couch. While it is more comfortable than sitting in a chair face to face, this form of therapy is more of an intellectual exercise than a means to change, as has been proven irrevocably by the famous and the infamous.
I believe that it takes approximately five years of practice beyond internship and classes to become an effective therapist and ten years to begin to approach being a master therapist. In that time, one learns what therapy is really about. Maybe it starts with good chemistry, but it ends up with good physics. You get on the same wavelength, the same electro-magnetic wavelength. This is partially a result of brain function such as mirror neurons and partially a result of hearts connecting. In fact, the human connection involves every cell of the body and is rare. Yet we are a species that has been given the ability to ride the same wave and we do not use it enough. As in many other kinds of relationships, the beginning involves chemistry; the duration involves neuro-cardio-physics. And that is what lasts.