Have a volunteer cut the deck into four piles.
Shuffle the cards using a complicated process, and astound your audience: The top card on each pile is an ace.
Prepare by secretly stashing the four aces on top of the deck.
Put the deck face down on the table.
Say, "It's hard to know when I'm tricking you and when I'm not. To show you that I'm not, I'll let you cut the cards for me." Have your observer cut the cards in half, making two stacks.
Say, "Now, cut that in half," pointing to the first stack, which has the aces on top. Put the new pile next to the original. Do the same with the other stack. You should now have four piles of cards in a row - and the pile on the end has the aces on top.
Say, "Let's mix them up even more, just to be sure."
Point to the stack of cards at the opposite end from the one with the aces on top. Tell your observer that you're going to do the "New Orleans shuffle." Play this up - you learned it from an old voodoo lady, etc. Pick up the pile and put three cards face down in the spot where the stack just was.
Now put one card on top of each of the other three piles and then return the rest of the pack to its original space, on top of the three cards.
Do the same thing with the next pile of cards, putting three cards into the empty space and one on each of the other three piles. Put the rest of the stack back on top of the three in the original space.
When you get to the stack next to the one with the aces on top, have your observer do the shuffle.
Let him do the last one as well. When he's finished, say, "Wouldn't it be something if, after all that shuffling, this top card is an ace?" Have him turn any top card over.
Say slyly, "Hey, you know what would be really crazy?" Turn over the other top cards, which are also aces.