ME! ME!
<hand waving frantically in the air> Call on me!
I've trained several dogs to ring a bell to go outside. Just hang it on the doorknob of the door that they go in and out of to pooty. It will ring naturally every time you go in and out, but you can give it a jingle to be sure that they notice it. It doesn't take long before they start ringing the bell to be let out. This is the hard part. They want out but don't always have to go to the bathroom. Have patience. Every time you hear the bell ring, RUN to open the door. Joey at first sometimes stood there and stared at me like "why did you do that?" That's okay. It's how they learn that bell ringing = door opening. The key is be consistent. Open the door for them every time they ring the bell. Before you know it, they're ringing the bell and going out to potty.
Good luck!