I have 2 ways of doing this trick:
The first is basicaly straight out of the box trick - lots of patter about an ID of card, then hand them a magical ID of cards, joke about them taking it out of the box and shuffling etc, turn over any card (I use lots of really blatent hints - chose any card which is close to your HEART - I even make it a joke, by saying 'heart' slower and louder - it makes the audience laugh).
T change the deck, I simple take the 'magic deck' off them, and lace it inmy pocket, then wave my hands etc and pull out the ID, claiming that sometimes decks materialise etc - then I reveal the card and they all go wild

The second way is similar to what was mentioned above - I keep a silk / hankey in my pocket, and when I remove it, I have the ID under it - the trick then continues as normal, but the deck suddenly apearing adds a lot to the illusion and IMO leaves a much stronger impression
Obviously the method I use depends on where I am, who Im performing in front of etc, hope this helps
Also, I have invented my own trick - based on the ID / brainwave deck - but dont have a name yet, see this link for details on it
http://www.talkmagic.co.uk/sutra183749.php#183749