Trust your developers. People tend to live up or down to expectations. Treat them well, and remember that loyalty goes both ways. After all, if you can't cut off thumb drives, you can't stop anybody from leaking code, no matter how much you don't trust them.
That being said, find yourself a lawyer with trade secret expertise, probably expertise in other parts of IP law, and ask how to legally safeguard stuff. You do want to make sure that, if a competitor gets your stuff, it's not legal for the competitor to benefit from it.
