Did you know you can specify which e-mail templates should be used for e-mail Replies, New Messages and Forwarded Messages?
Once you have your e-mail templates set up, open your Document Center by selecting Go To | Documents from the top level menu.
On the left hand side, you can set each Default E-mail Template, by right-clicking on the appropriate template, then selecting Set as Default, then selecting either "New Message", "Reply", or "Forwarded Message".
Now, whenever you Reply, Create a New Message or Forward a Message, the appropriate template will be used.
Pro Tip : E-mail templates are a great way to use HTML in your signature.