If you're going with a backpack, get one that has ventilation channels.
Once you start sweating, you can really feel the air moving in there.
But this will depend on your lean angle, I commute on a roadbike.
I tried a FUL backpack can't remember the model:

http://shop.ful.com/productdisplay.c...e=backpack&nt=

There are other brands that have the notches on the padding
that rests on your back.