From Robin William’s The Non-Designer’s Design Book (1994, Peachpit Press; ISBN 1566091594;Amazon | B&N.com)
Contrast
The idea behind contrast is to avoid elements on the page that are merely similar. If the elements (type, color, size, line thickness, shape, space, etc.) are not the same, then make them very different. Contrast is often the most important visual attraction on a page.
Alignment
Nothing should be placed on the page arbitrarily. Every element should have some visual connection with other elements on the page. This creates a clean, sophisticated, fresh look.
Repetition
Repeat visual elements of the design throughout the piece. You can repeat color, shape, texture, spatial relationships, line thicknesses, sizes, etc. This helps develop the organization and strengthens the unity.
Proximity
Items relating to each other should be grouped close together. When several items are in close proximity to each other, they become one visual unit rather than several separate units. This helps organize information and reduces clutter.