Dissociative identity disorder
There is no treatment approach that has been scientifically proven to work.  Most therapists who treat patients with DID use several different approaches.  Therapy usually lasts for several years, and does not always work.  Some therapists try to reduce or eliminate the alters so the patient only has one personality, while others do not.  The ISSTD recommends first improving a patient's ability to deal with symptoms and live more normal lives, then trying to deal with traumatic memories.  The final step is bringing all the identities together so the patient can access all of their memories.