It is the sebum - a fatty secretion produced by the glands on to the scalp - which makes hair greasy and if these glands are irritated in any way they increase their production of oil. So, if you have a tendency to oily, greasy hair don't wear your hair in a style that needs combing or brushing too often, don't wear a hat unless you have to, don't use really hot water on your scalp and keep out of the sun as much as possible. Greasy hair is easier to manage if it is kept short.If you wash your hair too often when you have greasy hair you will simply encourage the production of more grease. If you don't wash it very often at all you will forever be embarrassed by greasy hair. So you need to compromise. The best solution is to wash it every two or three days. Don't use water that is too hot or scrub at your scalp too much. Rubbing your scalp hard will simply stimulate the glands to produce more and more grease.
When buying your shampoo look for one that is cheap and free of additives. It is particularly important to avoid one containing a conditioner because your hair already has all the grease it can cope with. If you need a conditioner for split ends, apply it to the split ends and keep it away from the body of your hair. http://myshabi.blogspot.com/2008/10/things-to-know-about-greasy-hair.html