One-ahead principle is one of the basic techniques used in mentalism. Using it, you can make impossible predictions and perform some very convincing mind reading.
Let's have a look at a simple example of a trick using the one-ahead principle:
You ask your spectator to select any color he wants. Before he tells you what it is, you pretend to write down a prediction of his choice on a piece of paper. But what you really write is: "You will select the wallet." You fold the paper and put it in a cup on the table so the spectator cannot see what it says. Then he tells you what his color was.
Let's say he selected green. Memorize his selection and continue with the trick. Ask him to think of any number between 1 and 100. You make your "prediction" again, but this time you write: "You will select green." After the prediction is put into the cup, you ask the spectator what was the number he was thinking of. You memorize it and move on.
Finally, you give him a choice of three different objects: a pencil, a wallet and a watch. You write his previously selected number as a prediction and put it into the cup. Then you force him to select the wallet, using magician's choice.
When this is done, you take your predictions from the cup and they all turn out to be correct. Remember to mix the papers a little before unfolding them, so the spectator doesn't know in which order they were put into the cup.
For easier understanding, here is a video of a similar trick:
Of course, it doesn't have to be that simple. This principle can be used to create powerful demonstrations of mind reading and clairvoyance.
TIP: Instead of forcing the last selection, you can just guess what it will be. If you get it wrong, don't worry. An occasional mistake can make your mind reading even more believable, especially if you have already correctly predicted five other selections.