Hey, Saxman, it's all in the lip, more obviously on soprano than on any other saxophone. It will take practice and a good ear. You'll learn to drop your jax somewhat on the upper octave so that it doesn't go sharp. It's difficult to make yourself open your jaw when you're playing higher because it's natural to want to tense up. Just keep practicing. You'll get so used to what you have to do that it will (in time) come naturally and you won't have to think about it any more. Good luck!

That was supposed to be "drop your jaw"!