For many people, hearing a bell or "On your left" causes them to look over their right shoulder, and move slightly to the left.

Tom (@Noglider) is right - drive defensively, even on a bike. Maybe especially on a bike. I always assume a car's going to stop in the crosswalk, not before it, or look left but not right if making a right turn. If I'm wrong, I slowed for nothing, but I'm right often enough to have avoided accidents.