In a way, you answered your own question. Cultivate the habit of looking over your shoulder often... as though you were a fugitive and were being hunted down. Err... well... I'm presuming you're not a fugitive, but one never knows.

Anyway, because you're in NSW, I would probably advise getting a mirror, and *still* keep looking over your shoulder. You may need to experiment with types of mirrors. I've never used a helmet mirror, but have used a round Mirycle (?) and a Rhode Gear oval mirror, both mounted on the end of the bars. The Rhode Gear seemed pretty good both in terms of rearward coverage as well as not being too sensitive to vibration.

Be careful not to put your entire faith in a mirror -- still look over your shoulder before making a move, as you would/should in a motor vehicle.

The debate over mirrors thankfully is not quite so heated as helmets, earphones, bike lanes and shared paths.