One crucial aspect that I don't see people mentioning here: make sure you have at least one money sink that scales up automatically as people get richer. Fees for walking around the world and armor repair won't scale up, since they're based on fixed values.

Special novelty items that you can buy for large amounts of gold do scale up, in an economic sense, because as more money floods into the world people will buy more of them. However, this has a firm upper limit, and if way too much money floods in, they'll stop scaling.

The standard scaling-money-sink is auction/market fees. Charge a percentage of the purchase on every successful auction. If people have ten times as much money, then auctioned items will go for ten times as much thanks to supply/demand, and your money sink will pull ten times as much cash out of the economy. Since money sources are almost always constant, this can easily serve to keep a lid on the economy.

You do have to ensure that people buy and sell things regularly, however - trade skills and consumable items (including gear enchants and gems, in the WoW model) are necessary for this.