We all get spectators to sign our playing cards. We just can't help it. Funny really, as it's such a strange thing to ask a spectator to do. Magicians get so used to doing it during card tricks. It even becomes surprising when a spectator is shocked that we are about to ruin a playing card.
Make the playing card a point of contact.
Don't just get them to sign the playing card. When you take it back off them, make sure you quickly add your website address to the card under their signature.
Let's face it, you are going to give them the playing card at the end of your card tricks and spectators really do keep them. You want your contact details on that playing card!
Doing a magic gig, you will be handing out 20-100 of these cards through the event, so make each one count as a business card.
Make the playing card lucky.
You can also write 'Good luck' above their signature and 'From your name and 'your website' under it.
A playing card with a spectators own signature is nice, but adding your own, makes it an autograph. You might not be famous, but if you impressed them, they may well think you will be one day. Another motivation to keep the playing card safe.
They may discard it. But that's OK too. Someone else may pick it up and keep it. Even better, the next day, the venue or event manager might just find one, and now has your contact details!