I would like to add encouragement to all regarding curing / decreasing psoriasis. I've "had" Psoriasis for a little over 20 years, and have done a great deal of research in the more recent ones.
Yes, Psoriasis is curable in the sense that you can figure out what causes it in yourself, avoid that, and your Psoriasis will go away.
From what I can tell, psoriasis is definitely caused by your body's ability to handle its own food diet. Here are some things I've done and discovered.
Rosemary - I'm into herbs, and rosemary is an anti-inflamitory. I grow a rosemary plant indoors, and I discovered eating just one leaf (needle) per day caused many of my spots to disappear over the course of a couple of months.
Vegetarianism - I switched from a "meat eater" diet to a vegetarian diet about 4 years ago, and I noticed a near-immediate improvement in my skin.
Sunlight - I actually got a light sunburn on my forehead where I had psoriasis (back about 10 years ago), and when it healed, the psoriasis was gone from that area. It has never returned to that area. I'm NOT advocating sunburn -- it happened to me by accident while I was fishing :)
As Tom and others have so clearly stated, you need to find your "trigger(s)" by doing a short fasting diet followed by slow introduction of foods.
I still have a few spots -- probably a dozen tiny ones -- and this is due to laziness on my part. I'm vegetarian, but I just don't feel like putting in the effort to find my specific trigger(s). Someday, I probably will, but I'm fine with myself as I am at the moment, so I focus on other areas of my life.