The reason why I encourage studying the oft-neglected Mount and big sister Rear-Mount is defensive players expose their arms if they mess with your neck.
Those positions aside, realized after being stuck on Half-Guard bottom by very good player today that a sound bottom game is the inverse of a solid top game:
Get pressure on the top players neck!
Options as follows:
Pulling down on the neck @ 3:00 (think of this as a reverse cross-face of sorts, making sure you control the tricep with your other arm):
Driving bicep into far-side carotid @ 3:20:
Driving bicep into crown of their head @ 3:14:
Forearm into blade of neck @ 4:10:
Pushing palm into near-side carotid @ 12:25:
Overall, in deciding what strategy to use when, a sound choice will result in the top player creating pressure against themselves the more they insist on crushing you with a given approach.
Next, remember that a cross-face for example can be used to suck you in tight or create massive pressure, so the defensive measures for both will likely be different. Grab a training partner and play around, noticing what’s there and what’s not. The right move in the right context will feel like a magic trick, so keep experimenting.
Lastly, changing gears completely, I usually try to roll/reverse people as they’re transitioning to cross-sides from a guard pass and/or moving between cross-side positions. The momentum is there, it’s just a matter of MacGyvering it to our advantage: