Look out your windows and see where your eye is drawn. Plant them where they will be visible from a window. Once you have decided on the style and function of your yard you can begin picking plants. Two elements to consider are repetition and contrast. Repeating a plant throughout the yard will provide unity, but too much repetition will be boring.
Contrast adds interest and variety.
However, too much contrast is confusing and chaotic. Balancing the repetition and the contrast is where the art takes over from the science. Movement deals with traffic flow, how you will move through the garden. It also deals with how your eye will move through the garden, and how curving sweeps of plants and partially hidden views will draw interest. Depth is created by using low, medium, and tall plants, or by using various shades of the same color.