The classic means of controlling mudflation areLink (via Wonderland)
* reducing the amount of value in the economy via means such as: item decay, item deletion (either via challenges where you can lose items, or manually; yes, some games literally just go in and delete items), character purges, etc. Many games used to simply wipe the player database every few months because mudflation had gotten so bad.
* Refusing to up the level limit, and instead introducing orthogonal advancement paths. This single mechanism is probably the single biggest slowdown you can effect. Many long-running successful muds got that way by simply never upping the level cap, and instead investing their expansion in enriching the content and encouraging repeat play.
* Obsessive attention to economic stats and adjusting all economic drains to account for all the influx. This often means punishingly high costs for players, btw.
(Disclosure: I am a proud member of the advisory board for Areae, the company Raph founded)
Koster's keynote from Game Developers Conference
Koster's amazing "What are the lessons of MMORPGs today?">
Areae: online world startup from "Theory of Fun" Koster
Destiny of Games: what will become of fun?
Lifecycle of a gamer
What would an MMORPG about healing be like?
Civil liberties in gamespace
Theory of Fun: Understanding Comics for games
Star Wars Galaxies economy laid bare
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.