A small tilt off the hand could do the trick. The Deck is held horizontally, and as the packet is taken from the top to the bottom, tilt the right hand fingertips downwards and the right hand wrist upwards. Try to think of it as showing the audience the top of the pack. This should provide cover.
However, you must remember that the most valuable thing when performing the pass is misdirection. A quick hand-on-the-shoulder and a look up to ask what was their card is all that is needed. As soon as your gazes meet, Pass!
I always thought that a casual misdirection is better than a super quick invisible pass, since it will still arouse suspicion.
One last tip: stand in front of a mirror (or record on a camera if available) and spread out the cards. Then square them up again. Repeat this over and over. Now all you have to do is try and incorporate the pass into your natural squaring-up motion. Keep alternating between just squaring and squaring with a pass. Change it step by step. Soon your pass will just look like your normal squaring up of the cards. Less misdirection will be needed in performance if you can achieve this, since the spectator knows you aren't doing anything funny with the cards. Or so they may think...
