Try using two hands. I can drop from 4 bars to one in about 20 seconds every single time.
Why would I use two hands? The iPhone 4 goes from full bars to SEARCHING when touch in the spot lightly. My 3GS MUST be gripped with 2 hands tightly to drop 3 bars. I was using an iPhone 4 the other day. I held it naturally in my left hand. My skin just touched the spot. The pone went from 5 to SEARCHING and my call was dropped. Holding an iPhone 3GS i the same way has no effect. In fact, in all the different ways I hold my iPhone 3GS, I don't lose a bar. But if I hold my phone unnaturally, it loses a few bars, but the call IS NOT dropped. I tested the iPhone 4 and 3GS in the SAME location, and had the issue on the iPhone 4.