They key is to be comfortable riding with no hands. Learn to ride easily with no hands, pratice it a lot, then reacing into your jersey will seem trivial. Also if you can ride with no hands you can open anything up easily.

Otherwise, like was said above, move to the back or wait until you are at the back, and pre-open any wrappers. I don't like to pre-open things personally because I often take more than I end up eating.