I freeze mine in a ziplock bag full of water to keep them from freezer burning. I have some skins in my freezer that have been there ten years. I got one out a year or so ago, tacked it to a board, fleshed it and let it dry, it was just fine.
Don't put salt or borax on a skin you want to use for backing. You can wash the skin for about a half hour under warm water and remove salt or borax but it is better not to use it in the first place, freezing is better