Anything out of the ordinary works well.
"Awww, you remind me of my grandma... "
"Nice! Is that look back in style again? "
"That's pretty cool, but do you have the attitude to back it up? "
Omg! I have the same outfit! Although I have to say it does look a little better on you. "
Just get creative & don't gush over her at first.
But after a moment or two, you CAN genuinely compliment her.
Don't be too cocky, and don't forget to mix in a little sweet with the sour.