hey I came up another great suggestion. Do it to the person you know, but that person don't know you. Make his/her name appears on the back of the selection.
Like this.
you: did you pick the card that you wanted?
spec: yes.
you: did i force you to pick a card or whatsoever?
spec: no.
you: I will shuffle this deck and i don't know your card. have you watched any csi on television before?
spec: yes.
you: do you remember the episode when they found blood on a clean carpet with the black light.
spec: yes.
you: ok. i'm going to do something a lot similar and were going to experiment this together. i want you to close your eyes, imagine that you walk into room with a black light in your hand and you discovered your name glows in blood splatter on the clean carpet.
spec: ok.
you: here take this lighter and pretend this is a black light. i want you light it up and keep it like that. then i will deal each one of the card face down under the flame of the lighter and something weird will happen.
you: is that your name.
spec: oh my god! yes!
when the spectator saw the name glows like blood splatter and you turn over their card which is to be their selection.