make sure your pack is adjusted well. too tight across the chest and it can increase contact. too loose and you'll get more bouncing and friction. i like to keep my chest strap snug but not tight, and make sure my shirt underneath isn't getting bunched up or pulled tight against my chest, and i'm fine. the couple times i've had some minor chafing, some vaseline or moleskin fixes it (though if you have a hairy chest, moleskin might just be delaying the pain til later).
