Open-mindedness, not being prepared and stupidity are pretty vague answers. If you could go into more detail about what makes a good magic trick believable, then that would be awesome.
I performed a few tricks on my case manager and psychiatrist today and only two of the tricks worked. My case manager easily debunked this trick with the four aces, and my psychiatrist worked out my other trick partially with four aces (where the spectator is handed a 'shuffled' deck and cuts the deck into four places then takes 3 cards from top to bottom and 3 cards from top to the other decks over each deck) and they were constantly trying to work out what made the trick, the trick. So what is it exactly that makes magic believable? I should be posting this on a card/magic forum but I think it still relates to ones psych.