Easy To Master Crap [UPDATED - Now with "Additional Extra Notes!"]
Now, here's my pet peeve with this trick: magicians who do it the way Michael Ammar does it on Easy to Master Card Magic, Volume 1. Ammar tells you to count 4 cards of the spectator's packet before handing it to them. In my opinion, that allows for the possibility of trickery! Why not just stop being a pussy and let the spectator count ALL of them? Yes, it may take some spectator management, but I promise, it will greatly enhance the effect.
Extra notes: to prevent the spectator from spoiling the trick in this case, hand it to them and say, "Oh careful! Don't drop it!" That'll cause them to tighten up their grip pretty hard. And now that you aren't holding the packet at all, you can demonstrate with them (using the portion of the deck you have in your hand) how they should be counting the cards.
Additional extra notes: Now that Sgt. Raymond reminded me in the "Comments" section, you should look into making this a stage effect. It's very doable as long as you have the spectator counting aloud in a microphone the number of cards. Also, make sure the vanishes of the initial three cards are as visual as possible. You should also consider classic forcing a three after the first three travel; this way, the second amount of cards looks like it was freely chosen by the audience. Finally, remember to have the spectators to count aloud and loud - don't let the spectators ever suspect that there was anything dirty in the counting procedure.
Coming soon: An update on "Girl Magic"!