The guilds that would want to see achieves are one's working on HCs, at least that's what I think. So unless they're REALLY hurting for a particular class they'd want to know you have some experience.

Other than that I'd say try to impress some people from a guild you'd like to get into. I got my brother into my guild by getting him into a pug run when they were short, and he impressed a few of our cores who vouched for his app. If you see a guy from a guild you'd like to be in spamming trade for, hell, Baradin hold get in there and impress. It's either that or wait for Firelands.