I would go for one of the expensive shampoos you buy at hairdressers, they really go a long way - you use half of what you do of a supermarket shampoo. My girlfriend gets itchy scalp in the winter and so do I. She uses one from Kerastase (spelling?) I use a John Frieda, but there's also Redkin and other good ones.

Also, don't wash you hair every day and when you do, keep the water as cool as you can handle.