Rear view mirror
When we got our new tandem, I outfitted it with some Sprintech handlebar mount rear view mirrors. They look pretty stylish. However I never found myself confident that if I couldn't see something behind us, it was OK to move out into the traffic lane. I was always craning my neck around to look behind.
On last weekend's ride, I reverted to my glasses - mounted Take-a-Look mirror. I had forgotten how much more confident this made me and that I could easily monitor what was happening behind us. The Take-a-Look is pretty geeky looking but the improvement in confidence makes the ride much more pleasant.