I use a MOUNTAIN MIRRYCLE. It's a 3# mirror that you can, with some inguniuity, mount to the lief of your drop handle. It gives a good view.
The ones that fit on the older STI shifters are even better. Too bad Schimano "improved" their product without making any attempt to either keep the Mirrycle accessory or invent one Shimano could sell. Shimano has crippled a great product without providing any alternative. Shimano didn't make any money -- they just injured a tiny "competitor" and hurt their oun customers. Stupid.