I have been using an ursack for a few years now. Probably about 60-70 nights in the adirondacks. I follow the manufacturers instructions. The sack is tied to a tree at about head level using a figure eight knot. I try to find a 8-10 inch tree with a small branch at that level which i then flip the sack over it after it has been tied off. This helps keep it from sliding down. (It never had before, but it is one of those unfounded fears I carry).
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http://ducttapeadk.blogspot.com