One trick I used before is to have lots of little cards of pieces of paper with the letter s on them. Each time your student leaves off an S...you toss them a card...you could even have just a plastic cup and pop a card in every time they miss an S.
If you make it clear that they don't want to receive these cards (make it fun and negative...oooo no...Don’t give me a card!!) They will start to be more aware of it. You can also use it as a way to show progress...at the end of each class...count up how many cards they have...and see if they are getting less and less each time.
I also used paper clips in a cup for general mistakes. They start the class with 20 and every time they make a mistake I take one away. You would be amazed at how attached they get to the paperclips. "Noooo Diana...don't take away my paper clip!!!" It is quite hilarious...and competitive...and these are business people in their 20's and up.
Just my two cents.
