Top performers use some version of this. I have always been baffled by the attraction because people forget what cards they selected. And it happens on magic instruction dvd's all the time.

Baffled by audience participation?

I don't have a huge amount of people select... I've played with it a lot and I have found that 5 people are perfect for myself

Still strong but not so long where people start forgetting.

The public is not that smart.
most have short term memory loss.

One way I have seen others prevent this is by having the person remember the card and the person next to them. This way 2 people are remembering 1 card. I havent seen a person have trouble doing it this way. Ricky Jay does the same in his card selection routine.

^ This.

Have them write their name on the card.
Take a look at Dan Harlan's "Easy Collectors" routine.

John G had a great multiple selection routine during his penguin lecture. Also let the audience know (which can be great misdirection)

"Please remember the card, don't forget it or the ending of this trick will be pretty terrible"

When a lot of these routines were first performed/created people had a better attention level. Personally if I do it, I limit it to four and have them all signed.

Doc Eason's and Paul 'FASDIU' Cummins wrote a great book on the Multiple Selection. It answers a few of your concerns, Zombie Magic

Maybe have them take a picture of their card with their cell phone? Might work?