This page shows you the proper ways to markup subtitles, alternative title, taglines.
Using the right tag to markup is important, because otherwise search engine may not properly categorize your page. (aka SEO.)
Firsts of all, by W3C's recommendation, you should not use sub-section header such as “h2” to mark subtitle.
Here's proper ways that will be recognized by Google Search.
One simple way is to use a colon. Like this:
<h1>Lord of the Flies: on Human Nature</h1>
Another way is to use “span” tag for the subtitle. Like this:
<h1>Lord of the Flies <span>on Human Nature</span></h1>
A more elaborate way is to use the “header” tag, like this:
<header> <h1>Lord of the Flies</h1> <p>on Human Nature</p> </header>
You should use CSS to style them. See: CSS Tutorial.