This x-guard sweep to the knee bar has always worked like a charm for me.

Few details that make it work effectively. One is coming up with the sweep do not wait and then go. As your opponent falls you should already be turning for the knee.

Detail number two. This can be switched based on preference. As you can see in the video I will do both. When going for the knee bar you can ether drive your shin across to stop them from coming back into you or if you are fast enough you can just step the foot over.

Give it a try and let me know what you think!