I always get a rash on my shoulders where the pack harness rests. Is there anything to prevent that?
Lighten your load, for starters. Like blisters, rashes typically appear where pressure and friction meet warm, damp skin. Smear petroleum jelly on your shoulders, and wear a wicking synthetic shirt. If these tactics fail, try a different backpack, one with straps that rub you at a slightly different angle. And get it fitted by a professional.