I would do tongue trills sticky out tongue warm ups
Th and general tongue twisters
when practicing slow the section with the th in right down and almost.. shock them when it comes round to it to change the thinking pattern when i say shock them i just mean, make a big thing of it at that part in the line and build up from there the talent code is a good book to just support how to get people to consciously practise something in addition to repetition
Probably all the normal vowel shape exercises, i think its just practise ive had students that ive told time and time again.. but if they dont consciously practice it and change the habit it tends to stay the same
Language is another tough one, german and french and asian find the english or american accent pretty tough ive found.. so its just like they are learning a new language
if they are young its easier adult harder like… if i had learned to do the rolling r with spanish when i was very young it would have been better, not sure id even be able to learn it now
if anyone can think