Chambons work OK, I prefer them to sidereins as the horse gets the option of stretching down without sucking back behind the contact.
The first time you use one, just make sure it's very loose until the horse gets the idea. I used one on my horse and he got it quite quickly, the best part was that he then realised he could stretch down while being ridden which is great. It comes in really handy when they get the idea of stretching, you can alternate regular work with stretching while you ride, so they get a reward for doing well.