Customize default fields for new messages in Mail
When you create a new message in Mail, you will specify who the message is to and the Subject. However, there are a lot of other fields that aren't enabled by default. You can change what appears by clicking on the button in the bottom left of the toolbar and choosing "Customize..." from the list.
You can enable a signature drop down menu, a priority drop down menu, a Reply to field and drop down menus for choosing which account to send from.
For me, Cc and Bcc were already enabled, and I chose to disable them as I almost never use them.