I have recently become aware of the difference between the English and Japanese "sh" sounds, which I understand are formalized as the palato-alveolar fricative [ʃ] and the voiceless alveolo-palatal fricative [ɕ]. I am trying to correct my pronunciation from [ʃ] to [ɕ], but I can't seem to nail the right sound. I discussed it with some Japanese friends and they suggested that I pronounce it without opening my mouth as much (口の空間を狭くして) so that's what I have been attempting, but it seems like that has somehow resulted in し often being misheard as ひ (資料as肥料、代償as代表) and the like. I did some research online and it seemed to reinforce what I was doing; I read somewhere that the Japanese し is pronounced with a "flatter tongue," that is, the flat part of the tongue behind the tip contacts the alveolar ridge. But the flatter I make my tongue, the less sharp the "sh" sound becomes, and altogether it sounds even more ひ-like.
Any thoughts, tips, or materials out there that could help me out? If I move my tongue very close to the front of my mouth as I pronounce it I can produce a much sharper "sh" sound, but is this correct? I feel like I"m using the tip of my tongue more than the blade in this case but maybe I'm wrong? Any advice you could provide me with would be incredibly helpful!