Currently, you can not save an email template. However, you can select to use the last email that you created. This will allow you to create a new email beginning with the template that you had created in your last email.
Create a New Message
Select Last Email from the email templates.
Edit your last email and Save
Tip: Tired of scrolling? You can expand and collapse the email header information by clicking on the top right hand side of the email body. This will bring the email body up to the top and make it easier to edit.