Just found a post this summer by Small Axe BJJ on how to effectively control the hips. I haven't watched the videos yet... but wanted to share.
Here's a sampling:
"Combat Mount, High-Side, or “Rickson” (whatever you want to call it)- this position is a great strategy to employ on an opponent determined to elbow escape from your mount. Again, the secret to making the High-Side a valuable position is controlling your opponent’s hip movement. Watch how I demonstrate a method to tighten down your high-side."