Occasionally, I have used common chickweed poultice on spots of eczema. It is very soothing. I can not speak to psoriasis. But, I imagine it would be at least somewhat helpful since both conditions are forms of inflammation.
Other cooling herbs in poultice form might also help. Miner's lettuce, comfrey, aloe slime.
I learned recently with eczema that it is due to toxicity in the blood. Toxins build up in the blood and are concentrated in certain areas of skin. Anotherwards, eczema is a symptom of a stressed body and is often tied to stress or toxicity of certain organs, especially the liver. This means that it is important to also treat the stress and in particular stressed organs such as the liver.
I am personally working on getting my digestive system back into a healthy, harmonious state. Part of my treatment is to drink raw apple cider vinegar (Braggs live culture) in several cups of water several times a day. This has been helping to calm certain inflammations and help raise my stomachs pH. In my case, my eczema is likely an allergic topical response do to low pH in my stomach.
According to Rodale's Encyclopedia of Herbs, burdock decoction is helpful in treating psoriasis when drunk 3 ounces at a time, 3 to 4 times daily. The same book mentions tincture of Oregon grape as a treatment for skin disorders as eczema, acne, herpes, and psoriasis.
Herbs that help liver function are useful. Consuming roots of dandelion and burdock, or making them into decoctions has been helpful to me, also.
Thank you very much Fil. Burdock grows near where I live, I will have to try and experiment with decoctions of burdock and dandelion. From what I have read herbal remedies seem to vary greatly in their effectiveness to treat individuals with psoriasis. The dandelion roots should do a good job of cleansing some toxins out of the system.