if he sits on your chest like that, you won't be able to get out, so try avoiding him mounting you by hip escapingwhen he passes your guard!
We were drilling with resisting, starting from the mount. So he was already in mount. I would go for the hip escape and push on his knee and he'd plant his foot, then I'd try to turn the other way and he'd plant his other foot. Was frustrating and embarrasing because I know how to get out but couldn't.