Uneven spacing between heads and body copy, or between paragraphs, can be distracting to a reader. Subheads and short summaries or lead-ins can provide a transition between headlines and body copy. Subheads also break up body text into sections, and identify the subject of those sections. Do eliminate widows and orphans. Widows are very short lines, one or two words, at the bottom of a paragraph.
Orphans are short lines at the top of a column.
Usually minor editing, adding or cutting a few words, eliminates the line or pads it out to almost full column width. Do use clipart and other graphics to brighten up your pages, but don't go crazy with such art. You also want to be consistent with your art. Cartoonish clipart may clash with straightforward line drawings or illustrations when used on the same page or in the same section. It's usually pretty obvious even to non-designers when art styles don't blend well.