These are sounds that often lose clarity in speeches - especially when they are close together and a speaker might be going a tad quicker than they should. Here's a warm-up to help with that:
Sally, the sea shell seller, sells sea shells down by the seashore. If Sally, the sea shell seller, sells sea shells down by the seashore, how many shells does Sally, the sea shell seller, sell?
This one can be really tricky for those who aren't practicing this area of articulation. There is a lot of competing sounds, and the switching between the 's' and 'sh' sounds easily just come with practice. The jaw is a muscle too, and that muscle memory is as important for articulation as it is for typing or writing.