Borax in hot water approx 1 heaped tablespoon per one liter. Add some essential oil and iodine. Rub some of it into the scalp, let it sit for about half an hour. Then shampoo as usual. If it doesn't help, try some Selsun shampoo (selenium based) once or twice a month.
You may also add the borax solution to your regular shampoo, but for a "cure" it needs to work for at least half an hour.