We posted a bell by the door (hanging from a ribbon), and we'd touch Posey's paw to it each time we'd take her outside. We'd heap praise on her ('good bell!'), even if we'd helped her do it. Then we'd get very excited about going outside. ("yay, good bell! outside!")
I think it was just a few days (maybe a week), before she ventured over and....just....barely touched her nose to it. It didn't actually ring, but we heaped praise on her. We kept making sure she swiped it with her paw before we'd take her out, and she quickly picked up on it and started ringing it herself more often than we were ringing it with her.
Then—when she was about 7 months old—she started her adolescent rebellion phase, and started ringing the bell when she'd want to go out (yay!), when we were making dinner and she didn't feel like enough attention was being paid to her (uh oh), and when she wanted to go outside and find a stick/run around like a madwoman/check out the weather (ummm...it's COLD out there).
She was doing this at work, too—ringing her bell every twenty minutes, even if I knew she'd done all of her business, just so I'd play with her or take her upstairs to play with the other office dogs. After a few days of spending most of my time outdoors (when she'd ring it, I'd dutifully take her out, she'd get five minutes with me trailing her, urging her to go potty, then back inside), I'd take away the bell after she'd abuse it once, and put it back once she'd forgotten about it. Without a bell, she'd amuse herself or cuddle on my lap. Then I realized she was fine waiting for me to take her out, when I forgot to put the bell back after an hour or so, and she didn't have any accidents.
So it was her overzealous use of the bell that led to her transitioning to not needing it (at least at work—we still leave it up at home, where we've got a fenced yard and it's easy to let her pop out, even to play for a bit). She definitely knows it means the door opens, so it's been handy when we visit my mother-in-law—as long as she knows where the bell is, she's accident free.