There are other ways. I'm a big fan of Eve Online's PLEX system. PLEX is an in-game item purchasable with real money which can be cashed in for one month of play time. It does a great job of linking the in-game economy with real money.

Basically, PLEX is Eve Online's equivalent of a gold-backed currency. There's something real behind the play money, even if it cannot be extracted once committed.