DentonDoc wrote on Mar 11^th, 2011 at 5:48am:

Kingfisher wrote on Mar 11th, 2011 at 5:34am: I load mine with 30lbs of sand and set off on a fairly rugged trail. There is definitely some tenderness across the shoulders the first few times until the area toughens up for the season. I use the same idea, except I use left over plastic water bottles (refilled with tap water).  Its a bit cleaner than sand and you can add in known increments of weight.  And if a water bottle springs a leak ... well, its just water. dd

I started out with frozen gallon jugs of water and that worked pretty good especially in the summer heat when it functioned as a sort of back cooler. The only problem was that the ice/water was too bulky and I didn't have any room in the pack to carry anything else. The sand provided the weight at about half the volume and it conforms to the back better.