I suffered from psoriasis for 10 years, with nothing working, including both prescribed and natural remedies. Recently, I read a connection between psoriasis and candida. As far as I can tell, psoriasis can be caused by many things, which is why it's so hard to treat. If you haven't explored the possibility that you have an intestinal yeast infection, you may find that it's the source of your psoriasis.
I did the simple candida "spit test". Fill a tall glass of water & put next to bed. Upon awakening, gather as much spit as you can and spit into the glass before doing anything else. Wait 20 mins. If the spit forms long stringy 'legs' you probably have candida. If the spit sinks totally you could have severe candida. I tested positive.
I took Syntol (enzyme-probiotic) for 3 months to clear my candida (9 caps/day 1st month, 6 caps/day 2nd month, 3 caps/day 3rd month) and also ingested natural anti-fungal agents to help (fresh garlic clove, oregano oil extract, etc). I noticed within the very first week that my psoriasis was already getting better and within a month, it was totally gone.
A bonus tip: A few weeks into treatment I also discovered cold-pressed Castor Oil and rubbed it into my skin daily. This seemed to speed up healing and also took care of my dry and flaky skin in non-psoriasis areas.