Really like that bell idea. Mine were taught to "Say It" when they want to go out. They go by the door and stare at you, and whenever you notice them then they say what they want.
The best way I found to house train is just be very routine, and obviously let them after feedings and every hour or so if possible while training. I ha that Luxury when training my Stella who had lived outside until she was about 18 months. I was recovering from a back surgery so I was at home with her when I finally had a place that I could have her inside. She was pretty much accident free withina a week to ten days.
Beau who is Stellas offspring basically followed his momma, and picked up on it very quick too. The only thing with a small puppy is they don't even have the muscle development to hold it until around 12-13 weeks I think it is. The vet can tel, you and I think I saw the 3 for it on a Hartz site that was giving house training tips. Also a lot of positive reinforcement. I still go outside with them fairly often and praise them when they do their business. Not saying that they won't start learning a routine before that as I started working with Beau at around 6-7 weeks, and I haven't had less than 5 accidents in the house at age 3.5 since about 4.5 to 5 months. I really think with him though the mentor thing was very helpful.