Most mental illnesses are treatable, and dissociative identity disorder is no different. The disorder can't be cured, but it can definitely be managed. It can take a long time, perhaps many years, and requires an intense commitment to treatment on the part of the patient, but it is entirely possible for someone suffering from DID to lead a normal, fulfilling life.
Recovering from dissociative identity disorder involves seeking treatment with a mental health professional like a psychiatrist, psychologist or clinical social worker. Treatment methods vary, and may include psychotherapy ("talk therapy"), medication (typically for accompanying conditions like depression or anxiety) and hypnotherapy. Some experts have found that alternate personalities may respond to the therapist under hypnosis, which allows not only the therapist but also the primary identity to gain information about the alters and what they have experienced.
One aspect of treatment that has changed in the last decade is the ultimate goal regarding the alters. Once, therapist sought to integrate them into the primary personality. They found, though, that this could trigger a survival response -- that the other personalities felt the therapists was trying to eliminate them. Now, the goal is to facilitate a more peaceful coexistence between each of the alters and the main identity.