One thing I have done is tell girls what they should've worn instead. I usually pick a small item (shoes, bracelet, earings, etc.) and tell them they look nice, but it would have looked better if they wore something black, different shape, etc. It works better for girls you've atleast known awhile. Whatever you pick out it actually has to work. I told a girl wearing all dark clothing and white shoes she should have worn black shoes. The response I get from interactions like these is that the girls tend to try and impress me and the hidden compliment about them looking good keep them in a good mood.
As a bonus the next day when shes picking out an outfit she is going to remember you critiquing her and adjust accordingly, or at least just remember you. This is a small, but effective, moment. I can't remember or find Chase's quote but he talks about how a girl simply thinking about you is putting in some sort of effort which leads her to chasing, trying to impress, or whatever. Hope that makes sense.
