Update: Mount Attacks Technique Video
This is a very simple and fast arm lock for when your opponent tries to escape the mount by throwing his legs around your torso.
You just slide your body up and around whichever arm you want to take and make sure your knee slides over and past the head keeping your knees pinched tight.
Trust me, after getting caught a few times with this your training partners will realize there is a reason why people say pushing when mounted is not a good idea.
Arm Lock from the High Mount Escape