Shoulder shrug, hands palms up. I just shrug my shoulders and look very repentent to show that I am sorry. He waived and took off. This answer is not going to win me the vest, but it is an honest approach that works very well most of the time for me. If there's time, I might shrug my shoulders, in a guilty childlike kind of way.
Courtesy afterwards makes it all okay again most of the time.
Road rage is a scary and very real problem that I see getting worse. The other driver will respond with a wave in response indicating that it's not a problem. Winston Churchill signaled victory with it. The flower children signaled love with it. I think most people wouldn't get upset if you cut them off as long as they know you are sorry.