Q: Do you have any techniques for teaching Sh for a client who can do a correct S?

There are several easy things that usually work well if the client already can do an S.

1. Use a Sequence: Have him make an S, and then slide into a whispered (voiceless) Y. Then have him do the same thing with the lips rounded. The sound of Sh often will be heard during the transition between the two sounds if the client can slow down and make the transition slowly. Teach him to hear the Sh that is embedded in there.
2. Use the Association Method with E. When you use the Association Method, you use the movements and positions of one phoneme to teach those of another. The tongue position for E is basically the same as that needed for Sh. If your client can say E, then do the sequence a-d below. Help the client listen very carefully to tweak the acoustic quality in order that he gets the best Sh possible. Make sure to do each step discretely. Have him:
   1. Say an exaggerated E. (Smile very broadly)
   2. Whisper this E. (The sound that results with not sound like Sh. It will sound like a whispered E.)
   3. Now round the lips as you whisper this E. (An Sh sound might come right then. If not, go to step 4.)
   4. Say E, whisper E, round the lips with the whispered E, and elevate the jaw slightly. (A better sounding Sh should come in.) Tweak the acoustic quality.
3. Use a specific oral motor technique: Have the client bite down on the back lateral margins of the tongue, push up the tongue sides against the upper molars (the “Butterfly Position“), smile broadly, and blow gently. A gross Sh should emerge if he can maintain a central groove while biting down. Then have him hold this position and round the lips. A good Sh should be heard. Then tweak the acoustic quality.

For more I have written on the sibilants, please see Frontal Lisp, Lateral Lisp.