How do get choked if your chin is in? Is his forearm/hand still in your neck?
Once it's on, your best bet is to go into an alligator death roll towards the open side (side of the choking arm) and hope the opponent will be weak on his followup. However, if the opponent is on point, he will take mount and finish from there.
Some ways to stop it while as the opponent is attacking it:
1. Step over the non-choking side leg as the opponent is attacking your head, finishing in sidemount. Even if he has your head, if you can start getting perpendicular, it's a lot harder for the opponent to get his securing arm in the proper position.
2. The other option is to make sure you can reach over the opponent's back before he lifts his supporting elbow up.
3. Another option that has worked sometimes against me is to focus on pinching your chin to your non-choking side shoulder. Even if the opponent has his choking hand in, it's hard for him to lock his grip because your chin will likely get in the way.
4. One more option is to use both your hands on the opponent's hands, looking to bend his choking wrist. If you can get the wrist to bend you will be able to break the grip and prevent the choke. Note that when you do this you will likely get swept because you have no base. It's still better than getting choked. This is often what I do when I'm deep in the choke.
But just like against any legitimate submission, if the guy sets you up well and you're caught, there's no escape. If it was easy to escape then it wouldn't be used.
