A helmet mirror is a good choice. A few riders here use the Cycleawear mirror, which mounts onto the helmet via a self-stick pad. It's on a ball-joint, so it's easily adjustable or removeable.
A lot of riders here use mirrors not for the city traffic, but because we have two lane roads with no shoulders. On a quiet country road, you can often hear a car approaching, but does the driver see you and is he moving over to pass? I like to know, either for peace of mind or for possible evasive action. Looking back with a quick glance just isn't the same, and in heavy traffic or in a tight pack of riders takes too long.