mepal,
Perhaps that is the misunderstanding- I would think it quite strange to make dress related comments, not to mention tznius related comments, all the time! Socially inappropriate!
A well-timed comment on an extremely rare occasion facilitates awareness and pride in dressing tznius- anything more is not only counterproductive, it would be odd. Brief comments of “I like what you’re wearing”, as you mentioned, are also helpful occasionally.
If you can make a comment directly complimenting the tznius aspect of someone’s clothing, and feel genuine sincerity, it is taken in the most productive way. The glow on someone’s face when being complimented in this way is exquisite, and the experience can alter one’s path. If you feel you are “in a play” when making such comments, they will not come out right, and be more of a turn-off than a turn-on. When in doubt, don’t, and stick with the generic. As in many areas, less is more! (Not fabric, that is :).)