Tea Tree oil is also great to wash your hair with.
I've found through experimentation that you need to put about 20 drops of Tea Tree into a 350ml bottle of shampoo. You might want to use a scented shampoo to mask some of the scent of the oil as it can be quite strong.
I found that after washing my hair with this a few times my hair became a lot softer and lighter and attracted less grease, but YMMV.