Tips for pronouncing X vs SH in Mandarin?
'x' is an alveolo-palatal fricative, while 'sh' is a retroflex fricative.

For retroflex 'sh', the tip of the tongue is curled up toward the roof of the mouth while the tongue body is low.
Mandarin 'sh' could be similar to Russian 'ш'.  Start with this wikipedia image of a retroflex stop, but make a slight space for air to pass above the tongue:



For alveolo-palatal 'x', the blade of the tongue approximates the palate while the tongue tip is low.  Mandarin 'x' could be similar to Portuguese or Catalan 'x'.  It can help to press the tongue tip against the lower teeth when learning to pronounce 'x'---this doesn't change the sound, it just keeps your tongue tip out of the way:



More details: Chinese phonology on wikipedia