Seeing a Wide eyed open expression, for me is the most effective. usually accompanied by shrunken irises too. the magnificient artist: P.Chronos has great examples of what i am talking about, to name one: hypnohub.net/post/show/634/
the expression just adds to the "stiff, locked, frozen, trapped"-ness which is hypnosis in its self, rather than a sleepy face which i personally think looks relaxing and bit comical.
I like all kinds of expressions! Here's a P.Chronos one I really like: hypnohub.net/post/show/55...k_skin-femsub-huge_breast I don't really know why though so I guess this isn't a very useful post...she looks very 'stupefied' I think.
Post #34643, Post #33443 (loli), Post #28597, Post #27943, Post #21864, Post #20414, Post #14863, and Post #14464 are some of the better examples from my favorites list.
I'm going to preface my examples by saying they're furry. The expressions that artist did were goddamned perfect for me. Makes me sad they've moved on from the whole hypno fetish thing.