I doubt there is really any standardization for bus driver/passenger intercommunication, but this is what I've observed of the protocol in place:
When a bus is coming which is not the one you want to get on, step back from the curb. There doesn't seem to be any sort of hand signal necessary to inform the driver that you don't want it to stop.
When the bus you want to get on is coming, move towards the curb or side of the street (but not close enough to put you in the path of oncoming traffic or anything) and hold your hand up. Do this when the bus is still at a distance at which it can safely stop where it needs to.
It's just as important to properly signal the driver from within the bus when you want it to stop. Try to ring the bell just after the bus passes the stop before the one you want to get off at, or one block before, whichever distance is shorter. Don't ring it more than once, or the fnords will eat you. If the bell is not working right, call out "Next stop, please".