Advice for Taking on a Co-Founder Once You've Gotten Some Traction Solo?

Has anyone here brought on a co-founder at a later stage once you've already gotten some traction solo?

I'm a solo founder, bootstrapping my business - a consumer marketplace with social networking features. So far, I've worn every hat from business development and marketing, to product development, finance and ops. I've made great progress with the help of advisors and contractors without raising any external capital. I have a strong pitch to put in front of investors and growth accelerators (eg Techstars, YC) for a seed round over the next few months.

If I can find someone who is the right fit, I would love to bring on a partner to take on the COO role to help me scale and get ready for a raise.