One of the tricks I was taught - and found it works quite well with /s/ and /sh/ is to insert an /h/ sound right after the /s/. Example: instead of trying to get "see", try "s-hee" or instead of "soo", try "s-hoo". Everytime I have tried this the child will imitate the h sound, leaving out the stop sound and before long they are actually saying the proper syllable!
I had a boy who used a stop /t/. If I asked for "see", I got "s-tee". If I asked for "s-hee", I got "see"!! Now he has no problem saying /s/ and /sh/ even at word level!
Hope this helps!