Of course many of us have restricted closet hanging space! If the sleeping bag is hanging up, be careful not to leave your clothing shoved against it after searching for a green shirt to wear tomorrow. <img src="/forums/images/graemlins/grin.gif" alt="" /> I would also reverse the hanging direction periodically to prevent the down from bunching up and would keep the bag covered with a sheet or something similar (breathable, not plastic) to keep dust out of it. It's even more important to keep the bag covered if you keep it under your bed.

I prefer to store mine in the cotton storage bag that came with it, on a closet shelf. I occasionally turn the bag the other way up. Note that if you live where it's damp in the winter (like my part of the country) it's a good idea to keep the closet door slightly ajar so drier air from the heating system will circulate a bit inside the closet. If the closet is against an outside wall (colder), I'd keep the closet door open all the way.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey