My dog,(Gordon setter/black lab cross) has very sensitive skin so we bought some psoriasis shampoo from our local pharmacy. We're now using ducray kertyol pso shampoo. See link http://www.amazon.co.uk/gp/aw/d/B0052EBNJ6/ref=redir_mdp_mobile Our vet is very happy with the dogs skin and fur.
My parents old dog,(English setter) had sensitive skin and she had to be washed with polytar shampoo. That's another psoriasis shampoo.