I've worked with several business partners and some were Great, while others not so much. The great ones were referred through my network of trusted individuals. Reaching out to people you know and trust and letting them know what you are doing and looking for is the best way to find a partner. The next step is to make sure you have shared values. Which means, you need to write down what your values are and then ask the potential business partner if they match with what his/hers are. That's the best approach I've found from my own experience.
Really have to use word of mouth here, there are people out there who will steal an idea and not give it a second thought.
Due diligence is key, find out as much as possible and ask for references.
Great Question. At Two Tall Totems (my company) we always recommend signing a mutual NDA before discussing any projects. This way we can move forward in confidence and have a legal understanding of who owns what. You can find template NDA's online if you don't want to pay a lawyer to draft one for you. There are also websites that are much like dating sites that help you find the skills that help make up the team.