Cleanliness/basic hygiene, decent haircut, nothing obviously skeevy about you is a good start. Confident posture without looming or staring, smiling, eye contact with her actual
eyes... all very beneficial. When a guy looks terribly scruffy or dirty that's almost universally going to put off a woman, and I know that it gives me the creeps when some guy sort of slouches or
ooozes over to mumble a dejected "hi" to me (you don't have to strut like a model but stand up like a vertebrate, at least!
)
Unless you are jaw-droppingly ugly you can probably dress alright and hold yourself comfortably and at least avoid actively turning women
off. The only other reason most women would make a snap judgment about you would be if you're stunningly gorgeous, but then you probably wouldn't mind. (Obviously, women often aren't interested in a guy
period, or just not then, or maybe they really hate your hair color or whatever, and there's nothing you can do about that. Approaching with a chill and decent attitude is a plus, and try not to take any rejections personally; bitter misogynists are
really unsexy.)
I'm not saying women won't check out a guy, or don't appreciate a sexy visual! But I think most women are realistic enough not to automatically reject anyone just because he isn't Brad Pitt. The snap judgment I personally make first is "is he acting like a skeeze?" (if he is then he gets shot down fast) rather than "is he super hot?" (which is less of a deal-breaker, because most grown women have realistic expectations!)