I recently trained my Boxer puppy to ring a bell and it was ridiculously easy.
I attached a bell to a string and hung it on the dog while my pup watched. He immediately walked up at touched it with his nose. I praised and we went outside and I told him to "hurry up." He didn't within a minute or so and we went back in. Later, he wen over to the door again and played with the bell. I did the same thing as before and he went this time.
By the third time, he just walked up, rang the bell, and sat down till we came over.
The next day, he would ring the bell once and, if we didn't respond right away, he would ring and ring and ring. The nerd!
It is VERY important that the puppy is already mostly potty trained. If not, it won't work. This is great for the puppies that are already by the door waiting to go outside and, when they do have an accident, it is near the door, implying that the puppy knew they needed to go outside.