Oily hair is caused by an overproduction of oils in the glands near the hair follicles. Many people find oily hair distasteful, since it often has a strange texture and it tends to get limp and lanky. Although oily hair is largely linked to genetics and hormones, there are some techniques which can make hair less oily. You may have to experiment with a variety of products and regimens, so taking notes can be helpful.
To begin with, you can attack the oil at its source, the scalp. Use an astringent like witch hazel on a cotton pad to gently pat the scalp, drawing oil out and removing it. Try to avoid getting too much astringent on your hair, as you can cause it to dry out, which is not desired. Combining this technique with daily hair washing can be highly effective.
In the shower, use a clear, simple shampoo formulated for oily hair, and rinse thoroughly. As a general rule, oily hair needs an alkaline shampoo, so look for a shampoo which lists its pH balance, or a shampoo which is especially for oily hair. Detergent like dish soap actually works rather well also, if you do not mind a few drops of detergent in your shampoo. In some cases, you may need to shampoo twice. Avoid conditioner, or use it in very small amounts on the ends of your hair if you start to experience split ends. Conditioner will simply coat oily hair in slick goo, which will make it seem even more oily.
There are also a number of rinses which work well on oily hair. Vinegar rinses are an old classic, but you can also use black tea, lemon juice, or even alcohol. Dilute the rinse in water, run it through your hair after shampooing, and rinse again to remove all of the residual material and odor. These rinses will help to draw out oil, keeping your hair brighter longer.
You should also brush your hair minimally, since brushing brings up oil and drags it through the hair. For people with dry hair, this is great, but oily hair does not benefit from more oil. For a quick fix, dust talcum powder over your hair and comb it out. The powder will absorb oil when you're in a hurry, although you will probably want to shower later. A combination of these techniques should help to keep your head less oily, and your hair much happier.