Being a condition which is only ever treated, rather than cured, psoriasis is particularly prone to natural, alternative and complementary remedies.
In terms of herbal remedies, there is an emphasis on 'blood-cleaning' herbs. In practice, this means herbs that are supposed to improve the function of the kidneys and liver. I'm not sure why this should be, but there according to Chinese herbal theory, psoriasis is due to excess 'Heat' in the blood. The Readers Digest Guide to Alternative Medicine recommends infusions of dandelion root, burdock and red clover. According to Mrs. Maud Grieve (in 'A Modern Herbal', burdock purifies the blood, while dandelion root improves the action of the liver and kidneys. Red clover is more commonly used for cancerous growths; presumably, because psoriasis and tumours are caused by the same mechanism (cells multiplying way too fast), the same remedy would help both. Jaques Veissid, in 'Folk Remedies', recommends drinking a stinging nettle decotion (no, it won't sting you) four times daily and applying it to the affected areas morning and night. According to Mrs. Grieve, stinging nettles are good for the kidneys and are an all-round Good Thing. Veissid also suggests drinking a walnut leaf infusion four times daily (NOT a good idea if you suffer from a nut allergy), while Mrs. Grieve talks about walnut leaf's "curative effect on eczema and other skin diseases".
As far as hydrotherapy goes, Readers Digest Natural Remedies (from their Health and healing the natural way series) and Veissid both agree that bathing in salt water can be useful. I remember reading somewhere that the Dead Sea water (which contains stupid amounts of salt) can help skin problems, so this makes sense. Besides anything else, salt is an antiseptic, so it won't do any damage.
Bach flower remedies focus on emotional problems, and so do not offer a practical way to get rid of the condition. However, the Readers Digest Guide to Alternative Medicine recommends crab apple for disgust or shame and willow for resentment of the condition. It also claims that impatiens or rescue remedy, applied externally, will relieve itching.
Aromatherapy suggests using bergamot or lavender in the bath or as a lotion. A white lotion base is recommended if the psoriasis is not too dry or flaky, but if it is very scaly, a vegetable oil may be better, and sandalwood essential oil may be used.
In general, sunlight and Ultraviolet light are widely considered beneficial (but use sunscreen: pointless getting rid of your psoriasis if you end up with skin cancer). Excessive alcohol should also be avoided; one study showed that young men who drank to excess were more likely to get skin problems. Veissid also recommends an ointment made from soot and lard, but the sheer disgustingness of that suggestion makes me think it might be better to learn to love your scaly bits.