The metal skeleton of your external frame pack can take a beating, especially the bottom part that's always meeting the ground. Here are two ways to keep it in tip-top (actually, bottom) shape:
Prevent dirt buildup inside the ends of a hollow, U-shaped frame by adding rubber furniture tips. The tips also provide extra traction on wet rocks and ice.
Wrap foam pipe insulation, a piece of garden hose, or duct tape around a full rectangular frame to keep it from getting banged up.