There's a Hole in My Pocket
The Effect: Borrow a coin and have it marked. You say that you are always losing money through a hole in your pocket and then you drop the marked coin into your pocket. "What's weird", you say, "is that the hole isn't in this pocket (the one in which you dropped the coin), but in this one!" You lift up your other foot and the borrowed marked coin is there!
The Method: You need an extra coin. (Well, how did you think it was done?) Most people have a quarter on them, so let's say you want to borrow a quarter. On an "off moment", secretly place the extra quarter under your left foot. (I sometimes adjust my socks and stick it under the shoe.) Have someone loan you a quarter and ask them to mark it in some unusual way with a pen.
Take the quarter and just drop it into your right trouser pocket. Shake your leg as if you are shaking the coin into the hole and down your leg. Then say, "What's weird is that the hole isn't in this pocket (the one in which you dropped the coin), but in this one!" You lift up your other foot and the borrowed coin is there!
Then - and here's where you've gotta have guts - pick up the coin with your right hand and place it (don't drop it!) into the right trouser pocket as you pick up the other coin with the same hand. (Just switch them.) With your hand still in the pocket, look confused and say, "I'm sorry, did you lend me this or was it mine?" He'll tell you it was his and you'll return his coin (the marked one). He'll of course, inspect the coin for the mark. It's gutsy, but it works.