I'm no expert on Bivvies as such, but I know a fair bit about tents in general and I'd recommend storing it in the loft, fully erect.
( C, Mon 25 May 2009, 11:18, archived)

right now I'm fully erect

( GrandmaOfShoes, Mon 25 May 2009, 11:19, archived)

Are you also in a loft? if so, there's a fair chance that you're one of my tents.
( C, Mon 25 May 2009, 11:21, archived)