Sometimes I mount my bike on the bus and so, often I have to carry my helmet in with me. I prefer to take my helmet off when riding the bus, but now I'm stuck with a helmet on my lap -- a helmet with a mirror -- making it more cumbersome to carry. I'd like to secure it to my bike so that I don't have to carry it in.

Any ideas you guys have for this? Ideally it would be something i can quickly undo when i need the helmet.