Several of the above answers mention some important ways to reduce inflation but there's a few important money sinks that deserve some more explanation:

• Economic exchange fees: These are fees from converting a player's wealth from one form to another. A prime example of this in nearly every game is the vendor sell price. NPC vendors will always pay less than a good is "worth", and in the process players lose a lot of economic value. Players also lose value by using the auction house (posting fees), or enchanting items. You could also charge a fee for mailing items (a twinking fee) without too much complaining I suspect. These fees are great because the player will normally 100% accept them as the price of doing business, as they mirror real world financial transactions.

• Death Penalty: Many many games (WoW included) take a tax whenever a player dies, often as item repair fees. These fees are great because they help provide players an incentive to not die while not being as prohibitively painful as XP loss or flat out item destruction.

• Developer-run auctions/lottos: I haven't seen too many games do this, but one way to automatically reduce inflation is to provide some sort of valuable auction that players can bid on, but which is actually run by the developer (so 100% of the value is removed instead of transferred to another player). For instance, you could auction off naming rights for a street in town to the highest-paying guild, or a statue placement, or anything you want. The best part of this is that the cost should automatically adjust to match available player wealth, with no effort on the developer's end. Kingdom of loathing has removed tons of money from the economy in a similar by using a lotto system (100 gold to buy a ticket, can buy as many as you want) for rare aesthetic items.