I think one useful psychological subtlety with the classic force is to say "Just take a card for me now.", rather than "Pick a card". "Pick" suggests they have some choice in the matter, and it invites them to take their time and actually make a selection.
The word "Just" gets across the idea of the selection process being a small and insignificant event. "Take" is a clear instruction eliminating any suggestion of choice. "For me" suggests that this is a favour that you're asking of them, a task they must carry out on your behalf, and therefore not something that they are expected to be creative with, or to necessarily fully understand. "Now" indicates that they must carry out this instruction immediately and without hesitation. Of course, these ideas are not realised by the spectator consciously, but in that brief milli-moment of reaching forward and pulling out a card, it tips the balance in your favour that they will take the one you're guiding towards their hand.
In addition, the fact that the instruction is worded in a slightly more complex way than "Pick a card", means they are using more of their cognitive energy in interpreting the instruction, which means the card is in their hand before they have time to question it. When they remember what happened, and when other spectators see it, it will appear that a free selection was made, rather than you simply handing a card to them, which is what actually happened.