For external-frame packs, heavy things go high and as close to your back as possible. Picture a line running from the pack through your hipsyou want the bulk of the load to be centered on that line, so the pack's frame can direct it to your hips. Otherwise the load may pull back at your shoulders, which results in poor balance as well as discomfort.
You'll want to experiment some with your particular gear set to see what works best. It's not possible to pack things perfectlymost outdoor gear has an odd shape and doesn't lend itself to scientific packing. Keep the load close to your back, and tightly packed so it can't swing around, and you'll be fine.