Did you know that when Minnie (my dog) wants to go outside to "potty" that she rings a bell? Someday it would be cool to install a lower doorbell of sorts, but for now it is a simple craft that is cheap and takes about an hour to make.
You need a length of ribbon (think about the height of your dog for this one), 9 bells, 9 o-rings, 3 d-rings, and either a sewing machine or needle and thread. The pictures below are pretty self explanatory.
To train your dog, all you need to do is hit the bells right before you take him/her out, every time. Show him/her that she can hit them with their nose, and praise her for "ringing" it, even if you helped him/her with that part. Just keep doing that. Now, just wait and listen.
The first time that you hear the little bell ringing while you aren't standing there will feel like a miracle! Praise them, but immediately take him/her outside.
After they have successfully done this a few times, reinforce it by not taking them out unless they ring the bell.
My Minnie started ringing it when she just wanted to go outside to chase frogs (her most favorite thing EVER), but we figured her out pretty quick. Since she doesn't bark, it is so much easier for us, and maybe her, now that we can hear when she needs to go out!