Sometimes having them g-r-r-r-owl like a bear with the gross motor movements also works. Sometimes using a jar with a tightly closed lid (eg peanut butter) and having the child try to open in while producing "er" is very effective. It facilitates placement and once the child "gets" where his tongue needs to be he/she usually takes off. Sometimes prompting them to keep the train (their tongue) on the tracks (maxilary molars) helps their awareness hense production also. Good Luck.