Feather plucking – not to mention other feather abnormalities – is a massive subject.  It is frequently a complicated, multi-factorial problem, which may be resistant to therapy.
Although general advice can be given, which will on some occasions result in a recovery, the majority of cases require detailed investigations which may take some time, cost a considerable sum of money, and will not lead to a recovery in all cases.

Past history
In tackling a feather abnormality or plucking case, a comprehensive history needs to be established, including such factors as: age of bird, species, duration of plucking, season of onset, previous therapy, previous investigation, husbandry (housing, daylight pattern, occupational therapy, feeding methods, nutrition, concurrent illness, exact clinical signs, duration of isolation etc.).

Commitment
At the outset it is important to assess the owner's determination and resolve to overcome the problem.  Even if an owner says yes, they want to fix it, if they are not prepared to follow advice – whether this be medical, nutritional or behavioural – the condition is unlikely to resolve.  It is unlikely there will be an instant fix, and the cure will involve considerable time and commitment from themselves, together with a large amount of 'psittacine understanding'.
If the case is to be investigated, then the following list of causes are considered.  As stated by Tracy Gwillim in a recent issue, the key is to rule out all medical causes, prior to accepting that there is a psychological problem.

Medical causes of feather abnormalities/plucking/mutilation

Allergies
(Typically food, but occasionally inspired).  Some clinicians, particularly in the US and Australia, believe that birds may develop allergies which lead to plucking.  Research work in this field is ongoing, but all indications are that this does occur at least in some birds.
Simply trying different foods is unlikely to resolve the problem.  If one suspects foods, then an exclusion diet should be used.  This means feeding the bird on a diet that comprises proteins and carbohydrates it is not normally accustomed to, for a period of at least six weeks.
All parrot owners will immediately realise that this is not realistic for most birds, as changing a diet so radically is not easy, and may even be stressful.
The better solution is an 'intradermal skin test'.  This procedure is similar to that performed on humans, and is currently being developed for use in parrots by an international team of avian experts, including Dr Pat MacWhirter from Australia, as well as myself and Nigel Harcourt-Brown FRCVS from the UK.  It is hoped this allergy test will be available soon.  The initial indications are that basic foods such as millet, sunflower, peanuts, as well as breathed-in allergens such as canary feather dust and grain mill powder do certainly cause allergies, amongst other things.

Endoparasites
(This is, worms or protozoal parasites inhabiting the gut).  These frequently cause problems, especially in Cockatiels, and should always be excluded in all species.  The clinician should perform a faecal examination in all cases, to exclude these causes.

Environment
Excess tobacco smoke or too dry an environment (often triggered by central heating), can lead to itching or poor quality feather growth, leading to brittle feathers.  These break, then the bird plucks them.  Those of us who do not smoke can instantly tell the following morning when we smell our clothes if we have been in a smoky environment the previous day.  At least we can take our clothes off and wash them – not so the unfortunate parrot.  No-one should ever smoke in the same air space as any bird.
Most wild psittacines are accustomed to life in a rain forest, where they would be rained on daily, so captive parrots will usually benefit from a light, daily water spraying.  An inability to bathe is a common cause of feather plucking
The effect of a continuously dry environment may be premature wear and tear on the plumage, leading to tatty feathers prior to the subsequent moult.  Poor nutrition during a moult will have the same effect, leading to brittle non-durable feathers.  A parrot will often attempt to remove poor feathers.  Low light intensity will also cause plucking.

Excessive Daylight
This can be a problem leading to a tired, irritable bird.  We all know the effects of being chronically over-tired, irritable and bad tempered, and in parrots this can lead to self-mutilation.
The bird should not be in a light environment for more that 12 hours a day.  If it is in a lit room for longer, cover it up.  It is often best to cover a bird at night, only removing the cover in the morning when you get up.  That way neither the bird nor the owner is over-tired as both get the chance of a lie-in.  The bird's cage should not be left in direct sunlight.

Metabolic/systemic disorders
These have often been blamed for feather plucking and on occasions are responsible for it – in particular with an under-active thyroid gland, especially in cockatiels.  However, any form of liver disease can lead to itchy skin and hence plucking.  Chlamydiosis (psittacosis) is almost certainly the commonest cause of psittacine liver disease.  Septicaemia and air sacculitis may cause feather plucking, due to pain caused from internal adhesions or general soreness.

Lead and Zinc
Low level, chronic toxicity (acquired from the environment) will cause feather plucking.  This is now recognised as a far commoner cause of general low grade illness and plucking than was previously realised.  In the US it has been shown that there is no safe form of cage wire other than stainless steel.
With the passage of time – sometimes even with a new cage – a pet parrot that habitually chews or sucks the bars of a standard commercial parrot cage will gradually pick up significant levels of zinc.  It is my belief that all plucking birds should be screened for zinc and lead.

Psittacosis
(Chlamydiosis or Parrot Fever)  This is a significant cause of feather plucking, and should always be excluded.  Diagnosis in itself may not be a simple matter and treatment will be long winded (typically taking at least 45 days), and on occasions not effective.
Testing for chlamydiosis is not straightforward and no one test is wholly accurate, but it must always be excluded as a cause of liver disease, which in turn may cause feather plucking.

Infectious dermatitis/folliculitis
These are infections of the feather roots, or the follicles from which the feathers grow.  Cause and effect may be difficult to differentiate – for example, the bird itching at its feathers or skin because they are infected, or is the infection present as a consequence of the plucking and physical trauma of plucking.  This is an uncommon cause of plucking, but the feather follicles should be examined for any signs of swelling or discharge by the vet.

Polyomavirus
('Budgerigar Fledgling Disease') and Circovirus (Psittacine Beak and Feather Disease)
These do cause the development of abnormal feathers, poor plumage and plucking, although the pattern of feather abnormalities is typically different from a normal feather plucking 'pattern picker'.
These viruses should be excluded where relevant.  With Psittacine Beak and Feather Disease, apart from feather dystrophy (that is, abnormal feather development), one of the first signs is the loss of the grey powder down (especially typical of Greys and Cockatoos) from the beak.  This is a result of the powder down feathers being affected, resulting in a beak that is black and shiny.

Malnutrition
This is the single most significant medical cause of feather plucking in pet birds.  It is reported that between 75 – 90% of all sick birds are suffering from primary nutritional deficiencies.  This is a very poor reflection on our parrot nutrition, and is an issue that should be addressed.  Malnutrition may be caused by dietary deficiency, a digestive abnormality or a lack of access to unfiltered natural light.  Feather plucking birds on a predominately sunflower seed based diet must be converted onto a better diet and receive vitamin A supplementation in the interim.
There is no value in placing vitamin powder on top of seed diets.
Sweetcorn, apricots and other highly coloured vegetables are excellent sources of vitamin A.  Vitamins may be given mixed with soft food, or with some treat, for example toast and honey, or alternatively dissolved in water (Daily Essentials1).  If in doubt, it is essential to look into providing a better diet and ways of achieving the change, even though this is not always an easy task.  It is important that birds are not forced onto a new diet unless they are healthy at the time, or the stress of change may precipitate clinical disease.  Diet options are a wet/chopped, mixed 'sprouted seed/vegetable/rice mix' or a pelleted diet.

Neoplasia
Feather plucking will often occur over the site of a skin cancer.  There is circumstantial evidence that the trauma of chronic feather plucking can lead to such tumours, although the plucking may commence because of the tumour.  In such cases the growths are most often on the underneath of the wing.

Only when all the medical causes have been excluded, and standard environmental changes have failed to lead to an improvement, can psychological causes by considered. 
Many of the psychological causes of plucking have been mentioned by Tracy Gwillim, but will include:
Attention Seeking, Boredom, Over-crowding, Environmental Change, Stress, Excessive Preening, Nest Building, Feather Clipping, Trauma, Sexual Frustration, Lack of Behavioural Training, Incorrect Flock Status, Situation, etc etc.
It is my belief that feather plucking cases cannot be tackled alone by a behaviourist without the previous assistance of an experienced avian vet.
Likewise however, after the medical aspects of a case have been addressed, often a vet is not in a position to perform a house visit and follow up amended training procedures on a weekly basis, so these may well be performed by an experienced behaviourist.