I've been thinking about this problem as well. Here's my idea. Well, it's not a new idea, but basically I'd cut out two strips of some kind of foam, perhaps something like a quarter section of those foam pool noodles, and somehow fasten them to the back of the backpack, effectively raising them off of your back, and thus allowing air to circulate between it and your back. I haven't tried it yet, but I'm going to give it a go this weekend, as soon as I can find a pool noodle, which is difficult to do this time of year.

Anyone else try something similar?