What is the best treatment for Obsessive Compulsive Disorder (OCD)?

Behaviour Therapy: involves experiencing the fearful situations that trigger the obsession (exposure) and taking steps to prevent the compulsive behaviours or rituals (response prevention). This type of therapy is a structured step-by-step technique tailored by therapists to suit individual clients.  Behaviour therapy assists an individual to ‘unlearn’ disruptive behavioural patterns.
 
Cognitive Behaviour Therapy (CBT): CBT challenges the person's thought patterns and behaviour. Cognitive behavioural therapists focus their treatment on assisting the person to modify the thoughts causing their unwanted behaviour.

Psychotherapy: This is a form of ‘talking’ therapy. It can help the individual to understand and cope with their feelings and the difficulties they experience directly as a result of the disorder.
 
Support Groups: Support groups provide support, friendship, education, understanding and information for the individual with this disorder and to their friends and family.

 
Pharmacotherapy: Anti-depressant drugs, which specifically affect a neurotransmitter in the brain called serotonin, are the most useful of pharmacological interventions. These drugs are not addictive substances and work on correcting chemical imbalances within the brain. This is thought to reduce patterns of compulsive behaviour.

Established psychological treatments
When it comes to psychological treatment, research suggests the most effective treatment is some form of Cognitive Behaviour Therapy (CBT). This would usually involve Exposure and Response Prevention (ERP) which means the OCD sufferer is asked to repeatedly confront the triggers of their OCD behaviour and refrain from doing any neutralising rituals.  For example, someone who compulsively checks locks when they leave their house, would be asked to leave the house and only check once. Someone who fears contamination and washes repeatedly, would be asked to touch something dirty and not wash, and to allow the anxiety to go down with time. While this may initially cause considerable anxiety, OCD sufferers soon learn that the anxiety goes down even when they do not ritualise. By repeatedly doing the ERP, they will create new pathways in their brain and the urge to check or wash will decrease.
 
Because ERP is a challenging task, often seeming too scary to even contemplate, it is broken down into manageable steps, sometimes beginning with delaying the ritual, or changing it in some way, before being able to resist doing it all together. Cognitive Therapy (CT) is often used before ERP to help reduce any anxiety about doing ERP.
 
Cognitive therapy with obsessional thoughts differs from traditional CT with “negative thoughts” as  instead of questioning the evidence for and against the obsession, (eg “I might harm someone”) and  trying to convince  someone they would not harm anyone,  it challenges the thoughts about the obsession ;(eg, “Having these obsessions means I need to take precautions because it could happen").  Learning to step back and observe obsessions rather than argue with them or try to stop having them is key to managing them.  
 
Newer psychological treatments
While CBT is a well established treatment for OCD, more recent developments in psychotherapy are also being trialled with some success. For example, Acceptance and Commitment Therapy (ACT) has been found to help with OCD. A new form of Cognitive Therapy, called Inference based Therapy (IBT) has also been effective with OCD. IBT sees doubt as the basis of OCD. Persons with OCD may doubt their senses (“Even though I can see it is locked, is it really?“ ) and themselves (”Could I be someone who sexually abuses children?"). IBT helps the OCD client distinguish between “normal” doubt and “obsessional” doubt and understand how they have been confusing real probabilities with imagined possibilities.  I have found both these newer approaches to therapy very useful to use along with ERP to help my OCD clients gain back control over their lives. For more information on IBT see this article.
 
Involving Family
Family members can get unwittingly caught up in the OCD. Attempts to help, while calming the OCD sufferer in the short term, may end up making it worse in the long run. Moreover, hostility from relatives toward the OCD sufferer can lead to a poor response to treatment. Thus, educating family members about OCD and how to assist with ERP is also very important. 

The Australian Psychological Society publishes a scientific review of evidence-based psychological interventions for mental disorders every few years. In the latest edition, on the evidence for OCD interventions, it states:

There is Level I evidence for cognitive behaviour therapy (CBT) and Level II evidence for self-help (primarily CBT-based) in the treatment of obsessive compulsive disorder (OCD) in adults. One study with a small sample size provided Level IV evidence for acceptance and commitment therapy (ACT). In the current review, there was insufficient evidence to indicate that any of the remaining interventions were effective.

N.B.: Level I is the highest level of evidence.

As set out in previous responses, Exposure and Response Prevention (ERP), Cognitive Therapy (CT) and Acceptance and Commitment Therapy are used in the treatment of OCD. Given the fact that there is more evidence about the efficacy or success of ERP, it tends to be seen as the gold standard.  That's certainly the stance of the International OCD Foundation.

I have found that the work of the neuroscientist Dr Jeffrey M Schwartz has been very helpful with clients who have OCD. In many ways, it's a variation on standard cognitive therapy in that it employs a metacognitive approach to obsessive thoughts and to related compulsions.  By metacognitive we mean, thinking about the thinking rather than the content of the thinking/thoughts. 

Dr Schwartz's four step approach to OCD is based on thirty+ years research. He wasthe consultant neuroscientist in the film The Aviator about Howard Hughes who had OCD.  This easy to follow approach allows the person who has OCD to see that the habits and rituals that are part of her/his compulsions are based on what Jeffrey calls deceptive brain messages.  Anyone interested in exploring this innovative approach can go to his website www.jeffreymschwartz.com or read his books, Brain Lock or You are Not Your Brain.

I hope that this helps. 

I will not go through the process, it was well covered by MHA and the other professionals. In my view nothing works alone but therapy in conjunction with meds and support produce results. I cannot stress enough psychotherapy's (ACT, CBT, metacognition) role in the treatment of OCD. In a nutshell, it is the water in a fish tank; it supports life but too little and too unclear and the fish won't thrive. The therapist is the regular maintenance personnel that keeps an eye on everything so that fish thrive.