Psoriasis is an immune disorder which I don't think there is a cure (not even being smart - just joking).
My experience is to stop its growth at its onset. I use an ointment with a 2% coal tar. I also use white vinegar to remove the scaly skin if there is any, before applying the ointment. Calamine lotion can help if you suffer from the terrible night-itch.
Key point is, keep your skin moistened at all times (glycerine is a good choice after shower) and avoid going to places with extreme dry weather. I would wear long sleeves shirts and trousers if I have to visit those places.