First off, for an English speaker, the initial sound in "xī" and "shī" can be said to be /ʃ/ since this is the closest in the English sound system (and we don't differentiate them). Now as we're learning Chinese, we should try differentiate and eventually we'll hear the difference between them:
When you combine /ʃ/ with a front vowel (such as /i:/ as in "she"), you'll get the Chinese /x/; when you combine /ʃ/ with a back vowel (such as /u/ as in "shoe"), you'll get the Chinese /sh/.
So these sounds start off the same way but since the position of the vowels influence them, make them different: for the Chinese /x/, your tongue pushes to the front towards the teeth; for the Chinse /sh/, your tongue draws back away from the teeth (and if you do it correctly, the tip of the tongue should curl up and backwards).