How do I force the phone to shut down while im insideit?

Hi.

I were doing a small repair on my iPhone 3GS, and just to check that the device worked fine i hooked it up with my old cracked up screen. But the screen was so damaged that it just lightened up all white, and didnt respond when i pushed down the powerbutton and slided my finger across the top of the screen.

So how do I turn it of properly when i cant use the touch screen? Im still waiting for my new screen..