Having the students raise their hands in response to me doing so quietly has been a HUGE success. The students all react instantly and they even bug one another to get quiet quickly. It's almost like they are thinking, "What's wrong with you? Can't you see his hand is up?" Life is SO much better!
In addition, while I was giving out the new instructions I had another thought. My classroom has a netbook cart with 34 computers. We are using them constantly. Because of this the students can become very distracted by the things on their computer screens. Therefore, when we do our procedure of hands up, their other hand is to close their computer so I can be sure that I have their full attention. This also has been working wonderfully.
Accepting that you have a problem is the first step to recovery and this was no exception. Yay change!