Every time I finish a piece, I photocopy it and stick it in my mini moleskine notebook. They're the perfect size - just small enough to fit in my bag.
I've filled three of these gorgeous leather-bound books so far and I always, always, always have at least one on me. That way if an opportunity to show people my work comes up (as it often does) I have a portfolio on hand. It's been invaluable.
Want a sneak peak inside?