The following is a brief overview of the basic principles of design that appear in every well-designed piece of work. Although I discuss each one of these principles separately, keep in mind they are really interconnected. Rarely will you apply only one principle.
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—it's what makes a reader look at the page in the first place.
Repetition
Repeat visual elements of the design throughout the piece. You can repeat colors, shapes, textures, spatial relationships, line thicknesses, fonts, sizes, graphic concepts, etc. This develops the organization and strengthens the unity.
Alignment
Nothing should be placed on the page arbitrarily. Every element should have some visual connection with another element on the page. This creates a clean, sophisticated, fresh look.
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, reduces clutter, and gives the reader a clear structure.
When gathering these four principles from the vast maze of design theory, I thought there must be some appropriate and memorable acronym within these conceptual ideas that would help people remember them. Well, uh, there is a memorable—but rather inappropriate—acronym. Sorry.
This is an excerpt from the book "The Non-Designer's Design Book". I strongly recommend that you read this book if you appreciate design whatsoever.