When I see any situation ahead of me that will force my moving out in the lane, the first thing I do is check the mirror to see what's behind me and determine my action based on that. When I see a truck approaching in the opposite lane, I check to make sure another one behind me won't be passing it at the same time I am. I was once touring with a girlfriend, who was riding ahead of me, and I thought two trucks were going to pass her at the same time on a narrow road and started screaming her name, but she was fine.

I also find the mirror helpful for dealing with yahoos in redneck country, that being the sort of jerks who like to take a slap at you as they drive by. When I spot a suspect car approaching, I ride a bit further out than I need to, then use the mirror to move in just before they reach me, precluding any reaction on their part.

One way to try to get additional room is to hang some sort of bright/day-glo fabric on the left rear of your bike, which I realize is more mechanically difficult (and perhaps unwanted, based on your self-concept) on a roadbike. When touring once I found one of the orange flags that road workers use, and fitted it across the top of my rack so the flag portion hung behind my left pannier, sort of stating that was how much room I wanted. I noticed that cars definitely gave me more space after that.