I use a bag of holding. It isn't the smallest thing to carry around, but it is nice to have room for other stuff, too (it has enough room for a weekend trip if I don't need a spare pair of pants).
Yep! - it's out of stock on ThinkGeek but available on Amazon.
I have the bag of holding which is just a realy cool messenger bag, there used to be a verison that had backpack straps as well but they replaced it with a full on backpack of holding. I've heard you can sometimes find the hybrid version on ebay.
Edit: found a pic
Edit: Found it on amazon
is that a bag of holding? http://www.amazon.com/The-Bag-Holding-Messenger/dp/B003MAFFHS my brother has one and it fills up similarly