Using a bell alone may not be enough to get the attention of drivers. If a driver is listening to loud music or talking on a cell phone, there is little that a two ounce bell can do. In these cases, it is good to first ring the bell a few times to be polite, then, if the situation requires it, yell "Hey!" at the top of your lungs.
I can't tell you the number of times I have needed to do this in urban riding situations -- sometimes for myself, and sometimes for other people who hadn't learned this essential skill. Practice yelling "Hey!" late at night in an industrial district. Go to a sound proof room and practice it. Whatever it takes. You need to have an extremely loud, staccato "Hey!" at the ready. Once you practice it, you'll have it forever. So do it! And get yourself an Incredibell so that you won't have to yell "Hey!" at that nice old lady walking her Chihuahua.