^ Most of my collaborations have been with friends... which, honestly, isn't always the best way to go because I've lost friendships as a result of trying to collaborate with them. But if both people are on the same page, tend to think alike/agree most of the time, and are equally committed to the project and dependable, it can work out great. You just have to know your friends as writers before you decide whether or not you would mesh with them on a collaboration.
I've also gotten co-writers by asking, but the only time that I did that was for a story that was already in-progress, Code Blue. Basically, my original co-writer bailed on me, and I knew I wasn't going to be able to continue the story all by myself, so I asked for interested co-writers and ended up recruiting like six other people to help me, and that became my first big collab project like that. Some of them I knew as readers or fellow writers, but I didn't know most of them very well, so it was a risk, but it paid off for awhile; it was fun while everyone was into it. That's actually how I first got to know Rose.
Rose recruited writers for 00Carter by asking here, but I think the reason we got so many was because there was already an idea, so people kind of knew what they were getting into when they signed up. I think it helps if you have an idea to propose, and then people can decide if they're interested in being part of it or not.
There are writers who post threads here from time to time asking for co-writers, but I don't know that they get much of a response... It's kind of a gamble. I'll be honest; I would never agree to collaborate with someone unless I knew them well as a friend or writer, or I knew they had an idea that I was interested in writing.