“ch”, “j”, /r/, /s/, “th”, /w/:
http://www.sn.schule.de/~msweixdorf/dokumente/tongue_twisters.pdf
https://suite.io/lindsey-zenor/3ch62y5
http://www.write-out-loud.com/dictionexercises.html
http://www.speaklikeastar.com/2008/08/speech-training-improve-your.html