Should you be at a bus stop where multiple busses stop, and a bus begins to approach, but it's not the bus you plan on taking, it's courteous to shake your head 'no' at the bus driver, so he no longer has to waste time stopping for someone who will not get on. The next tip, however, is of even more importance:

Do not make any sort of signal to a bus driver you want to stop for you (other than maybe taking your bus pass out) Don't wave your hand or make any gesture. My girlfriend once did this, and the bus driver took it as a signal to pass us by. Oops.