The structural problem is that writers often use too many levels of headings. This leads to increasingly desperate attempts to make them visually distinct, usually with injudicious combinations of bold or italic, underlining, point size, all caps, and first-line indents. The result is trainwrecks like this:
5(a)(1)(iv) The Target Market
Has Expressed a Preference for
Copious Banner Advertising.
Headings are signposts for readers that reveal the structure of your argument. Note that I didn’t say the structure of your document. Headings that announce every topic, subtopic, minitopic, and microtopic are exhausting. If you write from an outline, that can be a good starting point for your headings, but don’t stop there—simplify it further.
Limit yourself to three levels of headings. Two is better. Readers should be able to orient themselves from the headings. With more than three levels, that task becomes hopelessly confusing. You may know your argument inside out, but no one else does (or will).
Once you’ve cured the structural problem, work within these typographic parameters:
Don’t use all caps. If your headings are full sentences, then they’re too long for caps. And Always Avoid Title Case, Because Your Headings Aren’t Titles.
Don’t underline. Why? Review underlining.
Don’t center, subject to the exceptions in centered text.
The best way to emphasize a heading is by putting space above and below, because it’s both subtle and effective.
Use bold, not italic. For headings, bold is easier to read than italic and stands out better on the page. And since the choice is bold or italic—not both—you should prefer bold. But even then, it’s an option, not a requirement. Non-bold headings work too.
It’s fine to make the point size bigger, but just a little. Use the smallest increment necessary to make a visible difference. If your text is set in 12 point, you needn’t go up to 14 or 15 point. Try a smaller increase—to 12.5 or 13 point.
Only use two levels of indenting, even if you use more than two levels of headings. Some writers like to indent every heading a little farther. Bad idea. It ends up looking random and messy.
See hierarchical headings for a contrarian suggestion about how to number headings.
If you’re using bold in your heading, you can also try reducing the point size by a half or full point. If your font has a relatively heavy bold style (like Times New Roman), reducing the size can offset the effect of the darker color, giving you subtler emphasis.
Certain web-design pundits claim that modular scale—that is, multiplying the body text by a recurring ratio—is a useful method of sizing web headings. I disagree. Yes, mathematical tools can guide certain typographic choices (see grids, for instance). The risk with these shortcuts is that they encourage typographers to satisfy themselves with numerical justifications—I used the golden ratio, therefore it must look good!—at the expense of developing visual judgment. When your headings look right, they are right. The ratio is irrelevant. (FWIW, I’ve never used a modular scale to size type. And never will.)