When approaching a traffic light/intersection where you intend to go straight and cars behind you may be turning right or moving to a right hand turn lane, I think it helps to quickly glance over your left shoulder and look behind you. The movement of your head helps to attract the attention of the driver hovering behind you; it also helps you gauge the "body language" of the car coming up to prevent the "right hook."

I think hand signals are great but there is not a generally accepted signal for "I'm going straight MF, so don't try and cut me off." And when making hand signals turning your head to the intended direction helps to indicate your intention.