Hey guys... this is my first post in this section, and I just want to share an idea I had and field tested earlier today with great success.
First take a pen and its cap, which has to have a tail coming off it. Twist this tail back and forth until it snaps off, which it will pretty easily if its one of those ten-cent pens. Now take a duplicate (yet unbroken) pen cap, put it on the pen, and keep the broken off tail in your pocket.
When you perform - and this is just how I did it today, there's likely a better method - have the tail in fingertip rest. Pull the cap off and hand the spectators the pen. Then drop the tail inside the cap (this isn't necessary, but it allows you to show your hands very cleanly). Tell them you'll twist this tail off and then mimic the action of doing so. When it's believable, let the tail inside the cap fall between your fingers, and pluck it off. Show the cap in one hand (concealing the tail) and the broken piece in the other.
Ask for the pen back and put the cap back on - its best if they're the same color, I suppose; I did it with a blue pen and blue cap (Papermate). Show the tail on the fingertips of your left hand and point to it with the pen in your right hand. The poiting end of the pen is the cap, the position of which should obviously conceal the fact that its unbroken. Do your striking vanish, and then hand them the pen.
I don't know how good this is, but I feel that it's a pretty magical idea and I was pleased with the reactions I got today. Try it out and let me know how it worked for ya. Thanks for reading!