So, I'm pretty sure that this is obvious to most people, but just in-case you haven't thought of it already. You can have up to 50 items in your completed auction tab. Post 10 items you want to keep "stashed" and then cancel them. They won't go back to your in-game stash but instead into the Completed Items tab in the auction house. Whenever you need them, simply clear out a spot in your in-game stash and collect your item from the auction house.
This would be useful for early characters who haven't bought much room in their in-game stash. Or for mule accounts to gain an extra 50 slots (regardless of size)
I haven't seen a recent post about this so I figured I would put it out there for those of you who might not think of this.