In my process of teaching, I found that to pronounce “z,c,s” and “zh,ch,sh,r” well is a very difficult thing for many students. They usually pronounce “z,c” as “s”, and pronounce “zh,ch,r” as “sh”. Or they often confound the six sounds. The reason is obvious that there are not consonant sounds like “z,c” or “zh,ch,r” in their mother language. The question is prominent especially, when the sounds above spelled with “i”.
However, my method to deal with the problem is not complicated. Putting the tip of your tongue against your lower teeth can make you pronounce “z,c,s” correctly and putting it against your upper teeth can make you pronounce “zh,ch,sh,r” correctly. And “s” is corresponded to[s]; “sh” is corresponded to [∫].
It’s not so difficult, doesn’t it?