If you get caught with the Americana submission the first thing you have to do is keep your elbow from being pulled down to your side. Once the elbow is down by your ribs it doesn't take much for your opponent to submit you.
Next you need to roll onto the same side as the arm that is locked. So if your opponent is locking up your right shoulder then you need to get onto your right side.
Of course, the best thing is to avoid being caught in the first place. Here Rener goes over fundamental principles that will save your shoulder.
