you are doing a good job.i am a v&A trainer with a leading international call center in delhi.
there cannot be a specific way of dealing with the problem,i would have shown them the jaw movement with the placement of tongue in the mouth while producing the vowel sound AA ,with them placing their hand on their jaw.
Show them the difference with YE and AA ,with the placement of tongue and jaw.
give them words that start with A in their mother tongue ,so that they can identify the sound easily.
Needs a lot of practice,but they will learn.
Thats what my experience says.