Most women/girls have a favourite accessoire that they wear often and that is different from the current trend, a neclair, earings, a pair of shoes, a handbag etc.

Watch her stile for a view days until you find out what it is.

Then make a compliment on that specific thing. Note, you must speak the truth, nearly all people notice when you talk about why you like something and you lie. Look until you find something you really like (and not her boobs)