We use a bell on the door, too, and I recommend it if people are really having trouble with housebreaking. A word of caution, however.
Boxers, being the incredibly creative dogs that they are, can soon extrapolate the bell to mean "I want to go outside to play," "I want a drink of water," "It's time for dinner," "I want a snack," "Get off the computer and pay attention to me," or any number of things

Just be prepared that, once the dog is older and you know he can control his bladder, you might have to just take the da*n bell of the door for a little peace and quiet!
Three of my five use the bell for multiple meanings. The other two don't use it at all, and instead have trained the others to ring it for them (I am not kidding!). You really have to admire them....
Julie