This is key. With any problem, you need to be on your guard all the time. I don't mean tense and worried, but you have to be thinking two steps ahead and anticipating... keeping an eye out for dogs long before they get near. Watch his body language long before anything may happen. It may sound like a chore, but actually it can it be fascinating, but will teach you a great deal. You will learn to 'read' your dog.

If your trainers are behaviourists too, then I would start with them. What did they get you to do around the dog in class?