Don't forget an extra bra too. They're like your boots though - a good fit is important and not all bras are built the same.
I respectfully disagree. Having a bra that fits is more important than the footwear.
Jeanette Isabelle 
I'd slightly disagree.  Pick up any Nat'l Geographic and you'll find a lot of women w/o bras.  Granted, most ladies I know don't plan on walking around au natural, but point remains, a bra isn't strictly mandatory.  I've had more than one female patient ask me about bra-related issues (and I have no idea why - literally next door my boss is an OB/GYN) - which isn't quite the same as having them for myself, I know 

 - so this thread is of some interest to me.
Something else to consider for women - birth control pills.  You don't HAVE to have a period every month.  If you're already on some form of pill, why not stash a pack or 2 in your BOB. Skip the last week's sugar pills and start the next pack.  You can do several months like this before really needing to take a break.