Signature Tweaks was previously a lab that enabled users to modify Google's default behavior of putting signatures at the very bottom of every email. It has now 'graduated' into the standard Mail platform.
To ensure your signature is always located beneath your reply and above the historical email trail, click on the gear at the top right of the Google Mail window and select Mail Settings. On the General tab, scroll down to the section that houses your signature and check the box in front of " Insert this signature before quoted text in replies and remove the "--" line that precedes it."
Then scroll down to the bottom of the window, click the Save Changes button. Signature Tweaks will be enabled and you'll be returned to Google Mail.
If you have more than one email address, you can also use this section to set up a different signature for each by clicking the drop down menu and choosing the other email: