If design was a theory it would be C.R.A.P. Any design course will tell you there are four design principles to improve any document: contrast, , alignment, and proximity. These apply to images, font, color, size, layout, and all other desicions that goes into a document's production. Used collaboratively, these principles work to guide a viewer's eye, creating flow in a document that both differentiates elements from one another while creating a cohesive look.
Contrast can be achieved with color, size, or font. Black and white is a classic form of contrast, but spice it up a notch by including an accent color (like blue to the right). Differentiate text within a paragraph by italicizing or increasing the weight. Combining more than one form of contrast is even more effective and visually stimulating.
Repetition, as the term would imply, ensures that design elements are consistent throughout the document. Repeating design elements adds to cohesion and flow to the document by relating similar elements. Make sure all headings are formatted the same to establish a pattern. Patterns make it easier for viewers to identify similar items, increasing readability.
Alignment. Nothing makes the eye want to vomit more than the absence of lines. Aligning elements within a document creates implied lines, or paths, for the eye to follow.
Proximity.White space is good when used in moderation. Make sure similar elements are grouped together. Otherwise viewers won't get the connection. Headers should be close to the body they support. Images should be grouped with the text they enhance.