Contouring is an extremely powerful tool that will aid you in accentuating or down-playing parts of the face. This is extremely useful when it comes to making up different face types.
My third huge tip, learn the color wheel. You cannot live without it and becoming familiar with it will only make your life easier. My knowledge of the color wheel comes into play every time I mix foundations, pick eyeshadow colors, blushes, the list goes on. There are pocket color wheels available at most office supply and craft stores.