Ask HN: How to open source?

I'd be glad to hear some success stories on how you went about open sourcing a project.

In what stage of development is it smart to put things out there? How do you make it easy for others to contribute? Have you ever just randomly released something only to later find out it's catching on?

I'm personally more interested in the case of pet projects that become successful open source software, rather than companies opening their products, but this is interesting too.

I'm asking this because I'm working on something that I believe could be OSS and I want to do it right. Seems to me like it can be harder than launching a closed/paid service...