I only think that I have sat in coach with an open seat in first once or twice, if it is open, I get a seat in first (however, where I jump, first is normaly full). I have started in coach and have been moved to first a few times, but if they know that they have open seats up there, I have been assigned quite a few times. Like previously said, be very friendly, and offer to the FA any help they may need.
Here is a question to all the jumpseaters out here. I am always the last one onto the plane, but do you wait to be the last off the plane as well? I usualy wait for the CA to open the door so that I can thank him for the ride, but what do you do?