Heads up! Not all themes are compatible with the plugins listed on this section. Before using a plugin, make sure it is compatible with the theme by checking the "Recommended Plugins for..." page of your theme's documentation.
Subtitles is a very well coded and simple plugin that allows you to add subtitles under your post title.
Just install and activate it, and at each post/page you will notice a newly added field for the subtitle, right under the main title of the post/page.