A couple of tips
@ Try and do it on a group of people so at least one of them remembers the card
@Ask "do you have a good memory" if they say "yes" you can do the trick on them, if they say "No", do the trick on them or do it on someone with them
@ When they see their card, tell them "It is essential that you remember this card, remember what it is, say it in your head, remember what it looks like... do you have it, okay, keep repeating it in your head so you don't forget it."
@ At any point during you trick, just ask "do you remeber what you card was, don't tell me, just remember it, okay"...continue with trick
@ (As the above) If they say that they don't remember their card and have someone with them, ask that person if they remember the card (make sure they don't tell you), if they say yes, tell them to whisper the name of the card to their friend.
@If however, they say no, stop the trick and
a. perform it later
b. start again
@ If during the trick, when performing on one person, you ask them if they remember their card and they say no, start again or stop the trick.
Hope this helps