If you are writing long posts, break up the sections by using headings, small titles to highlight a change of subject. In HTML, headings are set by the use of h1, h2, h3, h4, and so on. By default, most WordPress Themes use the first, second, and sometimes third heading levels within the site. You can use h4 to set your own headings. Simply type in:
<h4>Subtitle of Section</h4>
With double lines before and after and WordPress will make that title a headline in your post. To style the heading, add it to your style.css style sheet file. For more information on styling headings, check out Designing Headings.