With my larger packs, I lower it onto my hips. But if the pack is shorter than my spine, I find it is more comfortable to have it high, with a higher center of gravity.

Edit: A true sternum strap goes from shoulderstap to shoulder strap. It might be that you have my problem *blushes* a lot of waist belts don't go around my middle so well any more. If that isn't the problem, and the belt goes around your abdomen, so long as it is below your ribs and diaphram, and isn't so tight you can't move things about in your belly, wear it snug to your abs.