Besides using my weight to control the shoulders, the other important element to cross side control and keeping your opponent flat is ELBOW CONTROL.
Any time my opponent starts to get on his side, I will always go to grab the bottom elbow and lift it to prevent him from being able to get belly down and back to his knees.
By using proper weight distribution and elbow control you can make it impossible for your opponent to get to his side.
Cross side control relies on two major elements, the weight distribution controlling the shoulders and ELBOW CONTROL. In this video I show how once the opponent is able to get to his side controlling the elbow is crucial to flattening him out.
With my normal hand positioning from cross side the opponent tries to turn to get to his knees, I simply bring my arm around his head and pluck his bottom elbow to flatten him back out.