I use a mirror, and tend to take quick glances behind me (in the mirror) every 10 seconds or so. I only look back over my shoulder occasionally except when I am getting ready to move over.

Just as when I am driving, I try to be aware of what is going on around me.

I think that cyclists who see nothing unless they are about to run over it are no better than people who drive cars and continually focus on their hood ornament like it is a pair of cross hairs.