Using the drag & drop editor's Save as Template function, you can save your mailing as a reusable template in your personal template gallery. Your personal template gallery is the one that loads at the top, labeled My Templates, when you click to create a new email.
To do so, get your mailing looking like how you want it to look each time you select it. Then click the downward-pointing arrow to the right of the Save button and select Save as Template.
In the example above, you can see that General Ledger is the template from the template gallery used as a base for this new template.
A popup box will prompt you to name your template.
It's best to save your creation as a template before content, unless you have headlines that you'll use in every mailing, for example. All text and images you've added are permanent parts of your new template.