The Speech Target. Use this to help your students understand how close they are getting to accurate production of their sound targets. Grab the file here: The Speech Target
The Good Speech Posture Poster: Use this to help your students isolate their speech mechanism from the rest of their body. Grab the image here: Good Speech Posture Poster
Tongue Sweep Exercise from Pam Marshalla. Eventually, you add an "ah" sound when making these tongue movements, and in doing so, the final and furthest back position is the placement needed to make an /r/ sound! Don't try it too soon, though!
From this book, highly recommend!