If you hold the controls in the same spot for too long, it will execute RTH, at least in my case, I fly the FC40 & P1.
I do the same thing with a membership card with a hole punch in it. It works REALLY well and can give a nice smooth shot. Before that I was using the prop-wrench, but it can (and will) scratch the plastic surface. You can also find on ebay an elastic setup that helps as well (never used them, but keep seeing them pop up). http://www.ebay.ca/sch/i.html?_nkw=phantom+3+yaw+controlOr you could cut off the top of a cereal box and put a hole in the cardboard.
Slip the hole over the control stick and hold it down (it might take 2 fingers and a thumb to hold it in position). Hold the cardboard firmly to the controller to maintain the desired control input.