Mail - Multiple Signatures?

In Mail : Preferences : Signatures, select the account that you want the multiple signatures associated. Then add the signatures via the [+] button, change the signature content, and either leave the name of the signature as signature #n, or assign a different name. Click the red titlebar button to save and close your changes.

Without leaving Mail, revisit Preferences : Signatures, and select the account that you want multiple signatures associated. To change to a different signature in that account, use the Choose Signature: selector, and set it to the preferred signature name that now appears on that menu.