RioNomad said:
You escape by not letting them secure their position, I.E. don't let them get set on top of you and hold you tight, and shrimping out. If you let them get on top of you and suck up all of that space you're fucked. It's significantly more difficult to escape once they are hugging tight to your body. Never let them get setup tight liket hat.
Truth is though, without a lot of hours grappling, it probably won't work for you. Reading a Wikipedia article or watching some YouTube videos isn't going to get it done.
Here are a couple good videos, but really you need to practice this shit a lot.
Rio's 2nd vid describes 95% of the escape methods we would use when I practiced Jitz with my buddies. One of them could routinely submit guys in the 225lb.+ range, at 185 lbs.
Nearly every time he got mounted he used some variation of what was shown in the 2nd vid.
The OP asked a self-defense question, not how to win a competition. Having the knowledge to routinely submit someone is rather useless if the attacker's friends are kicking in your head or stabbing you -- while you are happily submitting the initial attacker.
The object of a street fight is not to fight, but to escape the situation with minimal risk of injury or death (unless you are a glutton for punishment). Therefore, the object is to get off the ground as quickly as possible and escape the situation.
BJJ is one of the finest martial arts styles in the world in for a fair one-on-one fight. But life is not always fair. Be prepared for the rest.