I've been using Ethique St Clement's shampoo for years. It's the only shampoo that actually helps cleanse oily hair without completely stripping it, leaving it squeaky.
My hair is very oily - I used to have to wash it every night - but since using this shampoo bar, I slowly managed to stretch it to every second night, and now only wash every third night, using dry shampoo on day three.
If hair is really greasy, wash twice, and really work the lather into the hair, massaging the scalp well.
I bought this product due to having oily, fine hair and this seemed like the best ethique product to match that at the time. However I found it to work almost too well. My hair became very dry and strawlike, and given that I had seen warnings that this might occur so I discontinued use. I had more success with other ethique product's.
Tips: Buy ethique sampler boxes so you can try multiple options before investing in full sized products.