They actually make a dog door bell - shaped like a paw. The advantage of this is that it is bigger so your dog has more surface area to touch (I think it comes with an instructional pamphlet as well. Check out
http://www.jefferspet.com - they might have it). An alternative method (explained fully on the "Take a Bow Wow" (available at
http://www.dogwise.com ) video series) is to tie a small bell from your door handle to a rope with a dowl at the end and teach your dog to pull on the dowel, thereby ringing the bell. To teach this, you'll need to know a bit about operant conditioning and target training. A good book that teaches about clicker training, for example is "Quick Clicks" also avaiable at dogwise. You'll also need to teach the bell ringing behavior seperatly and then tie it together as a signal that your dog needs to go out. Although I've seen this demonstrated on tape and in person, I've never taught it to my own dogs. From what I understand, once your dog understands how to ring the bell, basically, what you want to do is have your dog ring the bell evey time you let him out to do his business. Eventally, the dog begin to associate the bell ringing with the going out and will begin to ring the bell without prompting from you.