I think looking over the shoulder is out - having a roof come down around your ears, even if you are wearing a firefighter helmet, is not conducive to maintaining great flexibility.

Gotta go with mirrors, then.

The bike I'm going to use as a commuter has drop bars, but is not equipped with bar-end shifters. So far either the bar-end mirror like this one http://www.universalcycles.com/shopp...&category=2176, or the MTB mirror here http://www.amazon.com/Mirrycle-MTB-M...bxgy_sg_text_b may be my best bet.