Find someone who shares your vision. You'll need to be able to communicate effectively with your partner to make decisions, set goals, and drive the business forward.
Don't duplicate yourself. Find someone who's like you, but not too like you. They need to fill holes in your own experience or expertise, but if you are too similar you might end up at loggerheads.
Sneak a peak at their Rolodex. Are they well connected? A partner with a strong business network, industry connections or client list can help increase the value of your business and improve your chances of long-term success.