Given a blackboard or white board, draw a steaming bowl of rice on one side of the board and a person with a head of hair with a few little bugs in it. Then flip a coin and make a sentence like: He likes lice or he likes rice or use whatever paradigm your working on. Don't smile or give it away if you want to toughen it. Make it depend on the result of the coin flip. And tell the students to point to the word they think they hear. I found when I just thought up a result, some students tried to read my body language. Sometimes, I think with success. That's why I introduced the visible coin flip. Do as much practice as necessary, have a small test: Students write 1-4 and write l or r depending on what they think they hear. Those students who get all correct get a sticker. These can be an award in and of themselves for all sorts of desired outcomes in the classroom, but I like to give pencils, erasers and notebooks to the top performers each semester. The students aged 7-13 really get into it. Some dim teenagers think they are too good for stickers, but usually they are just dim. I have used this technique with all ages. The stickers I reserve for the 7-13 year olds. I'm sure some middle school student classes could get into it, but I have encountered some that just didn't care until the sticker system started to feel like a farce to me.
Best of luck.
Jake Mac Lennan
English Instructor, Konyang University