Effect:
Prior to the performance, ask a participant to sketch a simple drawing, seal it in an envelope and keep it with them. Later during the performance, you invite them to think about the picture and after some concentration you draw something on a notepad. You query the participant to open the envelope and reveal the drawing they made. You spin your notepad around to show a matching drawing.

Secret:

Oh man!
I've see Criss angel and David Blaine do this.
Too bad I don't have enough credits - That's my favourite trick ever!