The phoneme R has quite the reputation. It is difficult for many children to say, although most are able to attain it by the appropriate age of 5. However, for those that don't, intervention will be required. The R sound, otherwise known as "the Dreaded R Sound" by many speech-language pathologists, can be very challenging as a therapy goal. I have actually been able to achieve complete success for all of the children with whom I worked who were tackling R.
However, when I worked with George, a child with a sever phonological disorder (R was just one of his many goals), I met my match. No tried and true technique, no strategy out there, no type of prompt, cue, nor feedback worked! I could not establish the "Dreaded R" with him, no matter what I tried. One cue I tried to use was for him to pretend to gargle, which he could just not do successfully. So, I got creative. I laid George on his back and use a medicine dropper and trickled a tiny bit of water into the back of his throat, eliciting a gargle reflex. This was the trick that did it! Through this exercise, he was able to "feel" and "hear" an R, and, in turn, produce it on his own in short order, which led to complete domination over the Dreaded R Sound!
That was many years ago. Today, George is out of speech therapy, speaking like all other children his age!