I buy the clear cases, THEN use clear pushpins on the cases, that way I don;t damage the figure at all.
But, if you're not using cases, I would probably push the pin in as close to the cardback as possible "pinning" it to the wall, but carefull not to use ecessive force in pushing the pin in so far it leaves a huge intestation on the back.
Someone in another forum had some great pics of sort of a shelf made kind of rsting the cardbacks in a "slot" kinda neat.