I'm fairly new to backpacking, so yesterday I went out for a short overnighter. During the hike my pack kept sagging down. The waist strap would keep slipping down past my waist and over the top of my butt. I was carrying about 28 lbs of gear and it felt really heavy. While I'm not in perfect shape muscle wise, I'm not in poor overall physical shape. It just felt like I might have had too much weight on my shoulders. Turning my head left and right to look around put some strain on my neck.

In short, I know part of the problem is that I need to build some muscle, however that wouldn't fix the sagging issue. What could be wrong and how can I fix it?

Thanks guys/gals!