[emile]

Ask the students to draw where they think the position of the tongue and the teeth should be. That way you can pinpoint where they're going wrong.

The 'r' sound does not involve the tongue touching the teeth, and both sounds involve some sort of vibration in the throat.

I generally use this strategy with any sound:

1 teach them how to make the sound by itself
2 practise the sound in various words
3 distinguish the sound from others using minimal pairs
4 use the sound in drill sentences
5 use the sound in natural conversation

Each part takes time, but the fifth step is obviously the hardest as it involves changing ingrained habits.