Luna rings a bell to go out, and Apollo is learning. (We've only had him for about a week and a half).
With Luna, we just had her ring the bell every time we took her out to potty. I didn't teach targeting... I just took her head with my hands and (gently) rang the bell with her nose.
Every single time she rang the bell, we took her out, even if the ring was accidental or she had JUST come in from outside.
Now that she's trained, she does sometimes ring the bell to just go out and play. I've just learned to read her body language to tell if she really has to go, or just wants to be outside.
We're doing the same thing with Apollo, but it's going quicker because he sees Luna doing it.
When we visit houses of friends and family and forget to bring the bell with, Luna will "ring" the doorknob. She makes the same motion with her nose as she makes when the bell is there... except there's no sound because there's no bell!