A friend of mine from B7 said that there's a custom to palm one's face during Kriyat Shma with shin-shaped hand. It looks like a Cohanim's hand shape, but it forms a shin-like shape. Who's custom is that?
I have seen many who do this in practice, and I'm almost sure that it's always to mark the name of שד-י. The only source I could find however is here, where the rabbi who is answering the question says that Rabbi Ovadia Yosef mentions this in his book Maor Yisrael.
In my opinion this is much more of a minhag that is passed by people seeing other people and liking the gesture, but there probably are other sources of course who mention this.
My Rosh Yeshiva, Rabbi Leib Tropper, instructed his talmidim to make a shin shape of the middle three fingers, but did not explain it. Later, a very learned chassidic/yerushalmi friend of mine told me that this, together with a folded up pinky and a 90-degree-bent thumb makes דשי, i.e. שדי, and I have seen many chassidim do so.