The most important thing when crying on cue is to make sure that the audience can see your face. Don't just bury you face in your hands. If you do that, you communicate less emotion to the audience. To produce tears, keep your head up, but widen your eyes and strain them. If you refrain from blinking for a short time, your eyes should produce tears. This doesn't work for everyone, however. Some actors opt to use eye drops. In a pinch, if you really need to cry but don't have access to eye drops, a careful splash of water might do the trick.