Have you tried using bells? For Izzie we've hung a bunch of jingle bells (really a xmas decoration) on the doorknob of the door we use to take her out. We started with ringing the bells on purpose when we'd take her out, then moved to bumping them "by accident" on our way out, and within a few weeks she was hitting them on her own. We discovered that hanging the bells higher rather than lower forces her to jump up a bit which equates to her hitting the bells harder and them ringing louder. Good luck!
Carlie, David, & Isabelle (10 month old blenheim ruler of the house)