Being humble is NOT a turnoff. (Not to me anyway...) It's when women go way past that when it gets awkward.

Ex:

Man: You look amazing tonight.
Woman: Thank you! It's just something I had laying around. It's not much.
Man: Well it looks amazing on you.
Woman: Thank you. It's really not a big deal, I mean....

It's better to just take the compliment. Maybe you're just a little shy but hey a guy thinks you look pretty!