Ketoconazole shampoo is an effective preparation for dandruff, and is sometimes helpful for psoriasis. The shampoos work best if rubbed into the scalp well, and left in for 5 or 10 minutes and then reapplied. They are safe for daily use but may irritate if applied more than twice weekly. Some people dislike the smell - try shampooing again with a favourite brand, and use a conditioner. More severe cases require scalp applications.
Alcohol based products are easiest but can sting.
Steroid lotions can reduce itch but don't lift scale very well. Creams with salicylic acid and coal tar work much better, but are messy. They should be left on for several hours and shampooed off later. Most people rub them in at night-time, washing them off in the morning. Unfortunately in many cases the scale soon builds up again, so the creams may have to be applied every day to keep the scalp clear.