The key to contouring success is to know your face shape straight from the get-go. Whether you have a round face, an oval (the space at the forehead, nose, and lips are all the same), a heart (the forehead and nose areas are larger than the lips and chin area), or a square one (more defined jaws), understanding your face's shape will guide you in your contouring mission.
Remember, darker shades create shadows and make things look smaller and sharper. Highlighters or blushes will do the opposite and make things look fuller. (So you can see how knowing your face shape will help dictate how you'll apply your makeup, right?) For round faces, for example, you'd contour the lower cheek (from the temples to the lower cheek) and under the jawline to elongate the face and create balance, while someone with a square-shaped face would contour on the jaw and temple, but not so much the cheeks.