There are times when you just can't be bothered to wash your hair, but you don't want to be seen with greasy, limp hair plastered against your head. An easy solution to that is to simply get some corn starch out of your kitchen cupboard, sprinkle it on your scalp and make sure you evenly run it through your hair.
Some people call this "dry shampoo," although I personally never understood that term, because I equate shampooing with actually washing your hair. Regardless, corn starch is a great, natural way to soak up the oil in your hair, and it even gives it volume. This is the same reason why corn starch is an excellent natural face powder to set your makeup. If you happen to have a spare powder brush, use it to sweep the cornstarch along your scalp. Just remember not to use too much and make sure you really run it through your hair to avoid looking as if you have a layer of white dust on your head.