Get a nice backpack instead of the fanny pack... Try the osprey atmos or aura 35 (I think that's the name of the female version). It's got an angle in the back that'll keep the frame directly off your back and you won't sweat so much.

Put the shoulders loose so most of the weight is supported by the hip belt.

Otherwise, deuter makes similar backpacks with the mesh back and a frame that arcs it away from your back. The suspension/hip belt is not as nice on the deuter.

The annoying part about that is if you stand up to bike up a hill or hit a big bump, the frame is a bit loose around your shoulders and it does bounce around a bit.