Why do women who flirt sometimes bite their lips. It's cruel!! Stop that at once
Honestly? "Did you change your hair? ["Yes, I usually wear it up."] It looks nice!" Works well. (I agree with toothless, "regal" works if that's how you normally talk. If it sounds like you're trying to crib from a Regency Romance, don't. Talk how you normally talk. I would guess from his posts that's a max thing to say. I don't know if most men could pull that off.)
SMILE. Eye contact. For men, 'easy confidence' works with me--friendly, confident, no need to be obnxious or try to come up with a compliment just for the sake of it. Really, I'm the biggest sucker who a guy who seems to genuinely be interested in talking to me and listening to me and obviously isn't worried about just...chatting.