If your hair looks greasy and you don't have enough time to wash it take a small amount of baby powder in your hands and run it through your hair. Start with a small amount and add more if needed. The baby powder is so effective because it sucks up the moisture in your hair making it not appear greasy.
P.S, try to purchase an odorless powder because baby powder has a strong scent. If there isn't an odorless powder available, spray some perfume on your body, or apply a scented lotion to your hands.
Hi Kelly, thanks for the tip!
If you are in a hurry and need your hair de-greased quickly, you can always rub a bit of cornstarch in it as well as baking soda. Both absorb the grease quickly and leave the hair dry.