I disagree with the advice that Paul Green's DVD is the best resource. In my opinion he over-complicates the force, and, in fact, hinders it's ease of execution. His technique gives you three or four different things to think about at the same time, whereas I prefer to just give them the card. Just do it. Hand them the card you want them to take. In my opinion, the classic force is as simple as that. If you do it in an casual, relaxed manner, and say "Just take a card", rather than "Pick a card", or "Take any card you like", then you should hit it about ninety five to one-hundred percent of the time.