From my recent posting, this has so far worked with one dog. Our Brady understood in 6 days, Abby....well, she's a little slow in this department.
At petco/petsmart, etc. you can buy small cowbells (this was actually near the hunting typ stuff but the leashes/whistles, etc). Hang it from the door knob at a level that your dog can reach it with his nose (i initally thought about the paw...but that can causes scratches in the door).
Each time they go out, we stand by the door with a treat tucked behind it...they automatically have to "ring" the bell to get the treat and HUGE praises. Occasionally as they are sniffing around they bump it or sniff it my accident and we make another big fuss, a treat and run them outside.
I have read about people putting cream cheese or peanut butter around the edge too so they lick it...Good luck!
We use old Christmas bells that hung on our door at one time.
I know when we were training Gunner, we acted like "bloomin idiots" when he rang it. We jumped around and make all kinds of fusses over him. We used the spray cheese in a can. It took little time for him to understand and now if he REALLY needs to go he rings it like crazy. If he just wants to go out and play, he will ring it only once.
Just make sure to be very consistent with it. If he rings it, he goes out. Then they finally understand. Good luck!
my puppy goes and sits at the door in the sun room. the only problem with that is if my mom is watching her and she's sleeping or something as she works nights. I was thinking of getting a bell though.