in a good arm triangle the arm that is around the neck should grab the bicep of the other arm, you then fold the other arm at the elbow so that the forearm locks your hand in place, this takes your biceps and grip out of the equation and means that you can easily choke out a much stronger opponent. The hand of the supporting arm should then be placed on your opponent's forehead, locking your arm position secure and pushing their head back so that the choke sinks deeper. The arm that wraps the opponent's neck should have its bicep against the side of the neck, this provides pressure on the carotid and jugular, shutting down blood flow to the brain and causing the enemy to pass out, the forearm of this arm should be perpendicular (at a right angle to) the guys neck for best leverage. To finish the choke use the muscles of your upper back to pull your shoulders back ,this effectively makes your arms shorter and causes a great deal of force to be transmitted to the opponent. there are a few tricks with turning this into the "rack" neck crank if its not sunk properly but that's kind of difficult to explain and you should just focus on the basic submission for now.
Once this move is sunk in there is'nt a hope in hell of escaping, in about 2-4 seconds you'll be unconcious anyway (there's a great Bas fight which shows how quick this can be). The best thing to do is to avoid allowing your opponent to isolate your arm in the first place.