Four guidelines for layout composition
From The Non-Designer's Book

When laying out the elements of type and image for an ad or graphic design, designers explore a wide variety of layout compositions: dominant illustration, type only, vertical, horizontal, decorative, minimal, etc. Although there are no rules, the following four guidelines can help guide you in making layout decisions so that the final solution will be more effective in achieving its objectives.

Group related items together.
Create cohesive groups of information, not scattered elements. Develop hierarchal relationships to guide the eye flow of the reader. 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. Be conscious of where the reader's eye is going: where to start looking, what path to follow, and where to end up. There should be a logical progression through the piece, from a definite beginning to a definite end.
Purpose: to organize. Other principles come into play as well, but simply grouping related elements together into closer proximity automatically creates organization. If the information is organized, its more likely to be read and more likely to be remembered.

Nothing should be placed on the page arbitrarily.
Place each element on the page to create a visual connection with something else on the page. This provides a sophisticated and strong cohesive unit. Unity is an important concept in design. To make all the elements on the page appear to be unified, connected, and interrelated; there needs to be some visual tie between the separate elements. Even if the separate elements are not physically close on the page, they can appear to be related and unified with the other information simply by their placement.
Purpose: to unify and organize the page. The result is like when you pick up all the toys scattered around the living room and put them into one toy chest. The room feels organized and more comfortable.

Repeat some element of the design throughout the entire piece.
You can repeat color, shape, texture, spatial relationships, line thicknesses, sizes, etc. This helps develop the organization and strengthens the unity. The reader can feel more comfortable with the familiarity repetition provides.
Purpose: to unify and to add visual interest. If a piece appears interesting, it is more likely to be read.

If two items (type, color, size, line thickness, shape, space, etc.) are not exactly the same then make them obviously different.
If they're not really different, they can create conflict. Conflict can be uncomfortable to the reader. Contrast on a page draws our eyes to it. Our eyes and mind like the excitement contrast provides.
Purpose: to create interest on the page (so it will be more likely to be read) and aid in the organization of the information. The contrasting elements should not confuse the reader nor create a focus that shouldn't be a focus. The reader should be able to easily understand how the information is organized, the logical flow from one item to the next.