I remember reading somewhere (maybe on PT, maybe not), that it is difficult to teach a "quiet" command, since it's hard to anticipate the barking...as with other issues that you're trying to train away bad behavior, you have to catch them in the act and immediately correct it. With barking, a "no" or "quiet" usually follows the actual action of barking, so it's harder for the dog to associate the command with the appropriate behavior.

The suggestion that was made whenever I read up on this issue last was to first teach a "speak" command, which then allows the dog to associate barking with a command, and then to add a "no speak" command.

I know that Belle seems to put little to no thought into what she barks at; it is purely reactive to whatever she sees. She does not stop to think about what she's doing. I have had limited success with making her stop and come lie down, but it still does not stop the initial barking at whatever she hears or sees outside. I have been trying to work on this myself, but it's difficult for me to be consistent in training, since I have to leave her alone while I'm at work. I will say that I never try to correct her barking while she's outside in her 'play pen' (kennel). It is a behavior I only want to control when she's in the house.