Thanks--I'll keep all your tips in mind. Btw, I don't know about others, but for myself, I often fail to pay attention to the oncoming left-turning cars (the same when I am a pedestrian crossing a street), as my attention is devoted to the cars behind or passing me.
Ignore the cars behind you. Do you have a mirror? The amount of time you devout to vehicles behind you should decrease some with one, or at least you are still facing forward and spending less time on checks to your 6. If you've positioned yourself to the far left of the lane or in the shadow of the big vehicle, you shouldn't have to worry too much about the left turning vehicles anyway.