The way my husband gets our puppy to ring the bell is to ask her to "shake" when his hand is behind the bells.
I agree with some others that as soon as she got the concept, she started abusing it. For a couple days all I heard was ringing!!
I would just open the door to my apartment and let her run in the (enclosed) hallway if she had already peed so that she knows ring bell = open door.
It's been about 3 weeks and she uses it 75% of the time to let us know she needs to potty.