The first way is through the official forums, both via the individual realm forums and the multi-realm guild recruitment forum. Often times this means making a detailed and professional looking post and ensuring that it's bumped to first page of the forum several times a day.
The second way is through unofficial sites and forums. The process is similar to recruiting on the official forums, but recruitment generally is more directed towards the site's (and ideally the guild's) audience. For instance the guild recruitment forums on MMO-Champion are a great place to find hard core raiding guilds. Our Guildwatch column is another great tool.
The third way is through direct friend to friend recruitment. "I know a great Mage, even though she often likes to steal aggro and tank for a bit," I say to my recruitment officers. They go and grab the mage based on my recommendation. Whole guilds can be staffed like that.
Finally, the fourth way to recruit people is via the guild recruitment channel in-game. This is the channel where people spam their short recruitment messages in order to get people's attention in hopes they'll initiate further conversation or go to the guild's website.
I've used the guild recruitment channel once when it first came out and got absolutely no results. After that I've never used it again. I've seen some friends recently use it for their guild, and they too are not getting any good results. After some discussion with them it got me thinking about this on wider terms. It could be that the server they're on doesn't respond well to the channel, or it could be the channel is just a waste of virtual space.
The readership here at WoW Insider is very diverse. We've got the most casuals of the casuals and the hardest of the hard cores here reading and participating in the site every day. So I want to ask all of you, o' dear readers, does the guild recruitment channel work?