On my weekly club ride I met a rider who was just getting on the bike again after being hit by a truck, and spending a year in recuperation. The truck's side mirror clipped him (hit his helmet) and he went down, breaking ribs, shoulder and pelvis, and puncturing his lung.
This is one of the more common cycling accidents, and in many cases it can often be avoided by using a cycling mirror (though some feel they look geeky, one accident erases the stigma). After trying several models, I came across the Take-A-Look mirror shown below. It clips onto sunglasses, is very adjustable, and unlike many other mirrors delivers a sharp image.
It's fairly easy to develop the skill of watching two things at the same time in your peripheral vision: vehicles approaching from behind, and parked vehicles on your right, for opening doors. Then adjusting your line accordingly.
http://www.amazon.com/Bike-Peddler-Cycling-Eyeglass-Mirror/dp/B001VTQNVO