Setting aside obvious sexy things like cleavage-showing outfits, lingerie, a healthy curiosity about other women, etc....
Suspenders. Suspenders are the sexiest non-sexy thing a woman can wear. Rowwwwwwr (making clawing actions with my hand).
Vests. I love a woman in a vest.
Jeans tucked into calf-fitting boots (not those clunky Uggs). Or long skirts with calf-fitting boots. Honestly, pretty much anything with calf-fitting boots.
Hair that is roughly jaw-length, and which curls around to expose the ears. Or even better, only one ear. It's even more sexy when she occasionally reaches up to tuck an unruly lock back behind her ear.
An upper lip that naturally curls into a small smile even she's not smiling.
Eyebrows that arch upward into a look that partway between worry and sympathy.
No lipstick, or at the least, not red lipstick. (Red lipstick is too overt.)
The best makeup is the kind where you aren't quite sure it's makeup and think that maybe she has fantastic skin. It shouldn't be obvious that it's makeup.
A voice that's a bit higher-pitched than one would expect, but not shrill.
A tight-fitting lacy top that goes up to the neck, like 19th-century schoolteachers wore. The lace and tight fit imply sexiness, but the high neck implies that you'll have to work for it.
Those little white Keds. They make any woman's feet sexy. Don't know why, but they do. It's like she's a secret ballerina or something.
Red horn-rimmed glasses. But that may just be a byproduct of always seeing Sabrina wearing those with matching red shoes in college. That was one sexy engineer, my friends, one sexy engineer.
Things that don't work.
Putting one's hair up in a bun, particularly if it's directly on top of one's head. Pretty much the fastest way to uglify yourself if you're a woman. No man likes a bun on top of a woman's head, and I don't think most men like a bun in back.
Blue eye shadow. Or green.
Those bangs that stick up in front. What's the deal with those?
Underwear showing. I don't like to see bra straps. I don't like to see underwear sticking out the back. Underwear as translated from Latin means "Worn under other stuff", hence the word "underwear".
My ancestors fought cave bears so I could make this post.