How to carry sunglasses, so that I can easily put them on or off? Has someone discovered a way to carry sunglasses, so that they can be put on or removed while riding? 

I have tried two approaches, and both have drawbacks.

Carrying in a pouch, attached to the shoulder ring of the backpack results in scratched lens, because of the vibrations.

Carrying in the chest pocket of a jacket is possible only when it is sufficiently cold to wear a jacket.

My backpack is large and with a lot of options for hanging equipment on it, so maybe some kind of attachable box or external pocket?
If you are lucky/careful with your combination of sunglasses and helmet, you can store them quite securely in helmet's vents, which makes it easy to put them on/off with a single hand:



(quite hard to find a decent picture of this, screen-capture is from the Vuelta a España 2013, stage 14)