It's possible that your pack isn't fitted quite right. You might try adjusting the suspension so the belt is further up the pack, forcing your shoulders to take on a bit more of the load. You might also wear different shorts or trousers, those that are designed to work with a pack. Mountain Hardwear's excellent Synergy Short ($78), for instance, have a "conical cut" waist that doesn't bunch up under a pack belt.
The last solution I have-and I mentioned this just a few weeks ago in another column-is to rig a tumpline. This is a cloth strap that runs under the pack then up over your head, making your head and neck bear part of the load. That's how people in Third World countries, where they don't have access to $400 packs, carry 100-pound loads. It'll take some getting used to, but it works. Recently, I even saw a pack with an built-in tumpline. For a good description of the mechanics go to: www.cmc.org/cmc/tnt/no950/tumpline.html.