u have to handle it like a 'game' of sorts...

"ok, i'm handin u the spades, but remember where the clubs is..."
"without lookin, where do u think the clubs is..."

At the start of the trick I will say to them that " this is not a trick. This is simply a test to see if my hands are quicker than your eyes. Dont take your eyes off your card and see if you can spot me switching them. Dont turn the cards over until I tell you and we will see if you were able to follow".

This will accomplish two things:

1) They dont think that this is a trick so they aren't looking for any funny moves

2) It reminds them not to look at the cards until the end

I have done this trick maybe 30-40 times and only the first person I did it to turned their hand over before I was done.

say:
"why don't you trust me"
and then let them look the cards have changed

as used by Blaine

Tell them at the very beginning: Now this trick requires that you memorize your cards or else you will be very confused at the end> Hand them the first card and ask them " If you had to bet your life on it what card would you say you are holding. Tell them O.k. lets see if you lived just this once. Get on to the rest of the trick and say by memory only what card are you holding. And immediatly afterward say and what card do I have. This way they will be concentrating on your card not theirs.

NO....Star Wars Bounty Hunter.............What were you thinking