There's an old self working trick that I've read in several places over the years, and apart from being relatively unimpressive in terms of effect, the method remains completely baffling to me, regardless of how many times I look at it.
(That doesn't necessarily mean it's clever, I guess. I asked John Bannon to explain the 'Trick that fooled Einstein' to me, and he simplified it, and again, and again and again, until finally it clicked. And to his credit, it's not easy to simplify 'you are just holding 22 cards... their number doesn't make any difference', so he did well!)
Anyway, grab 20 cards from your deck, turn 10 of them face up, and shuffle the packet as much as you like, mixing the face up cards and the face down ones together into a heap. Turn them over and shuffle them again.
Now, in performance, you would hide this next bit. Perhaps put the cards behind your back, or under the table, whatever.
With the cards hidden, count off the top 10 (you can reverse their order or spread them off, doesn't matter) and hold them in your right hand, and then flip the whole packet over secretly.
Now, when you bring out both packets, they will both have the same number of face up and face down cards!!!
Like I said - the effect is pretty 'meh', but the method, I just don't get, and if I don't get it, hopefully laymen have no chance!
Anyway, making an attempt to have a proper go at putting a presentation around this pretty non-killer effect, I came up with the following...
You have a pile of 20 business cards. Each one is white on one side, with YIN in black text, and black on the other side, with YANG on the other. You explain that YIN and YANG are the forces of light and dark, and have to be balanced perfectly - and even in times of chaos, have a way of staying balanced.
10 of the cards are YIN side up, 10 YANG side up, and both packets shuffled together by the spectator.
You take them back, and go through the process above, to show that despite the shuffling, the two packets are perfectly in balance with each other, with each containing the same number of Yin and Yang cards.
It might be a bit meh - but I'm certain it will stand up to any amount of repetition, at least - as I know the method and still don't get how it works...
(I can't imagine anyone would be mad enough to want to market this, but just in case, marketing rights are reserved!)