Hi,
For some reason unknown to me (I don't speak any language where this is supposed to happen) I pronounce /s/ as a voiceless alveolar apical fricative rather than a voiceless alveolar laminal fricative. When I try to pronounce the latter I feel that rounding my lips slightly more helps forcing the tip of my tongue towards the alveolar ridge. Any tips as to how you could learn to distinguish between these two ways of making the /s/-sound?