The red/black division works well but means no one can really look at the cards faces - if you want to (casually) show the faces of each half pack (so they can see they are all mixed) then here's an improvement. Divide the deck not into colours but into odd and even cards: A, 3, 5, 7, 9, J & K in one pile (all mixed), and all others in the other pile. Trick is then identical to the one you described but now your spectator/victim can glance at the cards without seeing any obvious pattern.