The latest dev diary over at DDO explains how guild renown works as well as the reasoning behind some of the mechanics. In short: do stuff, get renown trophies. The trophies can be found almost everywhere: random chests, loot drops from monsters, even as quest rewards. The more difficult the challenge, the more likely it is that you'll earn renown.
Once your guild reaches level 26, renown decay becomes part of the equation -- you must maintain your guild's reputation or it will fade away.
It's a fairly simple system, but explained well in the dev diary. Take a look for yourself, and start planning your guild's trip to the top of the renown ladder.
