Short answer: you can't. Before you start screaming, "but, but, but" at your computer (or smartphone), yes you can do a lot to put your data under lockdown (encryption software, firewalls, elaborate password and security clearance protections, etc.). However, to my knowledge there is no software program or piece of hardware out there to protect a company from loose lips.
Just ask Yahoo.
This week Yahoo did NOT announce it has plan to shut down Delicious and other brands, including AltaVista, MyBlogLog and Yahoo Bookmarks. However, it is. I got my information from a leaked internal use only presentation slide from a Yahoo employee meeting. It's on TechCrunch if you want to see it. Yahoo has since confirmed that it's not a hoax.
This sort of thing happens to companies large and small everyday. In fact, I wouldn't have much to write about if it didn't. You would think an Internet company wouldn't be too surprised that its employees would have the savvy and wherewithal to capture a sceen shot and upload it to the world. I'm frankly surprised that Yahoo is actually surprised (If they really are. Really? C'mon, Yahoo).
There's the old adage that if you really want to keep a secret then only tell one person: yourself. If you want to keep company information on the down low, then don't tell your employees (unless you have one really good and scary, heavily lawyered NDA. Even then...).
Instead of planning on what to do if sensitive corporate information is leaked, why not additionally have a transparency plan? Yes, some information has to be kept under wraps at all costs. But, why not conduct a secrets audit and measure what information really needs to be safeguarded, what doesn't and to what degree? What are your criteria for information that needs to be secured and under what time perimeters? Put your energy into protecting the information that comes with compelling reasons for secrecy. Let go of the rest.
I might add that Yahoo has already confirmed the Delicious and AltaVista story. It had to, under the circumstances. In the long run, does it make a difference that we all found out before the press release went out?