I tend to get an itchy scalp too. What always works for me is a sugar scrub-- a bit of sugar (brown sugar or the coarser brown granulated kind) in your trusty co-wash conditioner. I usually add a few drops of jojoba oil if I think it's a dry scalp issue (it often is) or a drop or two of tea tree or peppermint oil if it's more of an irritation/flakes thing. Definitely no more than a couple small drops because those essential oils can irritate the scalp in larger quantities! If you use a tea tree conditioner, like Giovanni TTT, you can skip the addition of the oil, of course.
And I second the use of natural products for a co-wash... for me, those tend to be the least irritating. (By natural I mean more oils, etc. rather than 'cones, propylene glycol, synthetic fragrances, etc.) Hope this helps!