Everything others already said, AND: don't wear a backpack. Get a cargo rack + Wald folding pannier baskets, and put your cargo there. Ride with a messenger ba

Everything others already said, AND: don't wear a backpack. Get a cargo rack + Wald folding pannier baskets, and put your cargo there. Ride with a messenger bag, if needed. Then you'd sweat no more than you would while walking (if cycling at an appropriate pace).