If you're storing it long-term in between hikes, definitely store it as loosely as possible, on a hanger maybe or loosely on a shelf or in a drawer.
If you're storing it during the day while hiking, again, less compressed is better for the life of the garment. Keep it in a stow sack where it has a little room to breathe, not so compressed that the stow sack becomes a 'brick' in your pack. It should be easy to put in and remove from the sack, no forcing required.
-Mark