I learned how to do a full deck equivoque when I was about 11, from a book Gyles Brandreth wrote for kids.
I can remember breaking one of the rules and repeatedly doing this trick for one of my cousins (who was about 21 at the time). She never had a clue as to how it worked (in spite of what you might think of as the obvious issue) and got more and more perplexed as each time her choices led to the card I had picked that time.
I don't suggest trying that particular strategy, though! I suspect my age might have helped conceal the method. Also, because of my age I was not over-thinking it, so I am going to endorse magicarp's comment that confidence is important.
The other thing I think is vital is the timing of what you say and sometimes exactly what you say. To depart from the advice of Bananarama and the Fun Boy Three, it
is what you say
and the way that you say it.