Balancing the pack load is one of the most critical skills in packing and the primary determinant of whether a day's travel will be smooth or marked by shifting, stopping, and re-packing along the trail. The fundamental rule of pack loading is that the two sides of the panniers must be within five pounds of each other in weight, and ideally as close to identical as possible.
Before packing, organize all the gear and food into two equal groups — both by weight and ideally by bulk — that will fill the panniers to a comparable level. A handheld scale used to weigh both panniers before hanging them on the pack saddle is standard practice and eliminates the guesswork that leads to unbalanced loads. Even experienced packers who can estimate weight by feel use a scale when the load matters.
Once the panniers are loaded and balanced, the top pack is centered and secured with a diamond hitch or other appropriate lash that keeps the entire load stable through the animal's movement. A finished pack load should sit level across the animal's back, with neither side riding higher than the other, and the top pack should not be visible leaning to either side when the animal is standing on level ground.