You will probably laugh at this one, but it works great. Put the sheet in heavy zip lock bag then sprinkle a tiny bit of corn starch or talcum powder in the bag. Seal the bag, give a few shakes then remove all air and reseal the bag.
The starch or powder makes the sheet and the inside of the bag slippery. In a tight packed backpack or PSK, anything rubbing/pushing on the bag will allow the sheet to move slightly with little or no friction and makes for less chance of creating a wear spot or a hole.
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Earth and sky, woods and fields, lakes and rivers, the mountain and the sea, are excellent schoolmasters, and teach some of us more than we can ever learn from books.
John Lubbock