Position on the Page
The Center
The optical center of an image is a bit above the geometrical center. Most people say that the optical center looks more "centered" and stable.

When doing a docative image like an action scene, a landscape scenery or a portrait you should avoid the center of the image. Putting your main focus off center looks always more interesting and dynamic. But when doing a 3D rendering of a character or a design concept you can put your object in the center of image, because this looks more informative and focused. You can also center your focus in images which are supposed to look solid, stable and massive.
Position Exercises
When making a photograph one has to move around to find the perfect position to capture the subject. Drawing an image is a bit easier. One usually starts with a bunch of thumbnails to explore the composition. The following simple exercises can be done by everyone regardless of drawing talent. You can even use squared paper.

Using portrait, landscape and square frames to position one rectangle (or square) in it. Vary the positions, the sizes and forms of the rectangle. This exercise can be extended with more than one rectangle and rectangles in different values.