Effect: You tell your spectator to pick a number and do some math... You then name the number that they are now thinking of...

Secret: Tell them pick a number between 1 and 100, tell them multiply it by 2, add 8 to it, divide it by 2 and subtract the original number, IT SHOULD BE 4!!!! Now, pretend to concentrate and then name their number to be 4

Works about 99% of the time