times when you think they look genuinely pretty. girls feel kind of uncomfortable and/or disrespected when they hear about their "smokin boobs" or "dope ass" or whatever. if you tell a girl you think a certain outfit or hairstyle makes her look especially pretty/ beautiful, or she just looks great that day, she will never ever forget it.
Eyebrows. Sounds crazy but the nicest compliment a man ever gave me was that I had nice eyebrows. It was something specific not just yup you are pretty. A compliment on something specific wins over a general comment wins any day. Also, a compliment on a day where she is wearing little or no makeup will go a long way. Tell her she is naturally beautiful.
The best compliments are when she's, say, watching tv or writing or whatever. And then you just notice how beautiful she is and say something-that's the most sincere compliment. I would say eyes, laugh, how special she is, but only if it's true. Whatever you compliment her on, shell be happy as long as it's true and sincere.