Besides open-sourcing your project and legislation, are there ways to prevent, or at least minimize the damages of code leaking outside your company/group?

We obviously can't block Internet access (to prevent emailing the code) because programmer's need their references. We also can't block peripheral devices (USB, Firewire, etc.)

The code matters most when it has some proprietary algorithms and in-house developed knowledge (as opposed to regular routine code to draw GUIs, connect to databases, etc.), but some applications (like accounting software and CRMs) are just that: complex collections of routine code that are simple to develop in principle, but will take years to write from scratch. This is where leaked code will come in handy to competitors.

As far as I see it, preventing leakage relies almost entirely on human process. What do you think? What precautions and measures are you taking? And has code leakage affected you before?