Subheads are small headlines that organize the body of the article. They break up the text, making the article seem shorter and easier to read. This doesn't mean that readers have short attention spans, but subheads can improve their comprehension.
Subheads should always be larger and darker than the body text and in the same typeface as the headline, only smaller. Place them just above or next to the column of text they introduce, and leave adequate white space between the subhead and the previous line of text. Left justified subheads look better than centered ones.
The darker typeface of subheads also adds contrast to individual pages, while their repetition throughout the newsletter helps unify the appearance of your publication.