practice, practice, practice, and tactile cues is what my ds did in speech therapy. Like for the S/snake sound, he would take his pointer finger and move it in a line below his bottom lip while saying ssssssss. During therapy, he and his therapist would have a worksheet with a bunch of different pictures that used the S sound. She would stamp, or let him stamp I guess, the picture when he said the word right. I think I have some other exercises also, but I'll have to dig out my paperwork. My last two dc both had speech.