Ask HN: As a developer, how do I work with freelance designers?

I am working on a small start up, and I am nearing the point where I need to hire a freelance designer to add some finishing touches to the web application. This is my first production application, so I am quite nervous about just about everything.

How do I work with a freelancer though - when so much of my code contains sensitive information or information a designer doesn't need to see. On the other hand, I happen to be using Django to power the backend, and I don't believe there is a way to have him work locally without setting up a development server.

What do you guys do? Should I just make sure the contract clearly lays out he cannot use/share backend code in any ways? Do I need to change all my backend API keys and sensitive information after working with him? Or should I cross my fingers and hope my payment API account doesn't get hacked 6 months from now...