Sorry you are having trouble with psoriasis, my Dad has always suffered with it on and off all his life and it has appeared a few times on my skin. I put it down to something I have ate.
Do you take any drugs which are known inflamatories, such as asperin; this could make your problem much worse.Try visiting Elaine Gotchall's site and read about the specific carbohydrate diet. THis diet is mainly used to cure inflamatory bowel disease, but has also cured Psoriasis in children who have spent years seeing specialists. It is a very varied diet and only seems extreme because most of us are in the habit of eating so much junk food.
I had two small flare ups of psorisis and have not had it again since, after taking Benadryl. It does seem to be connected to what you put in your mouth.