Avoid complimenting certain physical features. The thing is, if you build up a girl’s feeling of worth in a particular feature, in most cases that feature is going to go away over time and she’ll end up feeling worthless because it’s gone. So avoid complimenting butt, legs, etc. Eyes and hands are a safer bet.
Complimenting on physical features works even better if you tie the compliment to her personality or actions. For example, tell her that her eyes are beautiful and that you love how her determination shines through her eyes when she’s working so hard.
Don’t sexually compliment girls you don’t know well. This is the biggest no-no when it comes to complimenting girls. You will obviously see girls you don’t know that you’re very attracted to, and you may think that paying them a compliment is just a nice thing to do, but if you don’t know the girl then a sexualized comment is just going to make you look like a creeper