Use a physical symbol as an anchor for repetition. I use a parrot flag. Students learn both that the flag means it is time to repeat and that when there is no flag repetition is usually out of place.
One of the concepts I think it's important for young learners to understand is that 'repetition is not language'. I want children to assume that if I talk with them I am engaging in genuine communication. Some children seem to assume that they should just repeat everything I say. Perhaps this is because they see me as a teacher and themselves as students. The parrot flag is one tool to help children move beyond this pattern.
But the parrot flag is more than this. It can act as a 'talking stick'. It can encourage children to say something. It can be passed around the group and the group repeats whatever the holder says. This is powerful and funny.
See Parrot Parade as an example of a musical activity that takes advantage of the parrot flag.
The flag can be used as a conductor's baton. Focus on pitch and rhythm as well as clarity of pronunciation.
If adopting this technique it is important to be consistent and avoid asking for repetition without using the flag.
It is possible to use the flag with any age group, though usually adults do not have the habit of automatically repeating everything the teacher says.