GA /s,z/ are grooved fricatives: the blade of the tongue articulates with the teeth ridge, allowing the air to escape through a narrow tube-like groove running from front to back along the center of the tip and blade. The tip is raised in the direction of the teeth ridge. If you pronounce an ingressive [s], with the tongue in the position for [s], you can feel the cold air rushing in through the groove. Some speakers allow the tip to touch the lower teeth: the result is a much less sharp and piercing sound. This type of articulation is the rule in popular New York City speech. Other speakers allow the tip to touch the upper teeth, producing a co-articulated [sθ] or  for /s/ and [zð] or [ð] for /z/. The incidence of this kind of articulation, technically known as a lisp, is surprisingly high among younger speakers of GA.
For GA /ʃ,ʒ/, friction is produced between the blade of the tongue and the rear edge of the alveolar ridge, while the front of the tongue is raised in the direction of the soft palate. Again, the tip is near the alveolar ridge or touches it, but may touch the lower teeth in the pronunciation of those who also have a low-tip [s]. The air escapes along a much wider and less shallow groove along the mid-line of the tongue than for /s,z/.
The difference in articulation between GA /s,z/ and /ʃ,ʒ/ causes GA /s,z/ to be high-pitched hissing sounds and GA /ʃ,ʒ/ to be low-pitched, ‘dull’ sounds.
Although there is considerable variation, the vast majority of Dutch speakers tend to produce sibilants whose resonance is the reverse of that of the GA sibilants. This is due to the difference in articulation between Dutch /s,z,ʃ,ʒ/ and their GA counterparts. In Dutch, the tip is held down and usually touches the lower teeth. As a result, Dutch /s,z/ tend to be low-pitched sounds. For AN /ʃ,ʒ/, tip and blade are held down and friction is produced between the forward parts of the front of the tongue and the hard palate, the overall tongue shape being rather similar to that for the vowel [i]. The difference in resonance between the GA and AN sibilants can be summarized as follows:
hissing, hushing or dull,
GA /s,z/ GA /ʃ,ʒ/
AN /ʃ,ʒ/ AN /s,z/
GA /s/ and, to a lesser extent, /z/ will strike Dutch listeners as particularly sharp and piercing sounds. By comparison, Dutch /s,z/, which are frequently palatalized and may even merge with /ʃ/, sound decidedly ‘sloppy’. GA-like realizations of /ʃ,ʒ/ may be heard in Mary Dresselhuis-type accents of Dutch in words like huisje, kas(t)je, genre. Similar realizations occur in the pronunciation of Standard Dutch speakers from Flanders, and in French.
Speakers who merge /s,z/ and /ʃ/ in Dutch may need to pay special attention to the following contrasts:
/s – ʃ/ see – she; sin – shin; pussy – pushy; rust – rushed
/s – z – ʃ/ sip – zip – ship; C – Z – she; miss – whizz – wish; ass – as – ash