I have psoriasis on one of my elbows and I feel your pain.. it gets so itchy it bleeds, as well. Sorry you have it on your scalp, that sucks. :\ That Neutrogena tar shampoo is supposed to be good. They also have some oral medications now for some types of psoriasis. Any sort of decent cream or something like that, you’ll have to get a prescription for, but I imagine it’d be hard to apply on your scalp.
I’ve tried store-brand antihistamine cream and it didn’t seem to be effective at all. I use a good antihistamine cream (prescription from Italy), sometimes a mousse-like corticosteroid (can’t remember the name) if it’s really bad. I do have a tube of Dovonex, but it’s Vaseline-like and I can’t seem to apply it with any regularity.
The Dovonex is a vitamin D solution; some studies show that sunlight can cause psoriasis to clear up. UV rays and vitamin D seem to have an effect. You can go to a tanning booth or get a sun lamp for SAD and expose your scalp for a few minutes a day.