It's always better to compliment somebody on something specific that is within their control, and that you actually appreciate, regardless if it's a man, woman, hot chick, whatever.
Saying "I like that blouse" isn't really about her, you could be talking about the blouse designer. But saying it looks good on her would be better, or saying how well it matches her shoes or whatever.
Try this one: "you have really nice posture. You carry yourself with a lot of self confidence."
or "I like that blouse. it brings out your skin color very nicely."
don't worry about her realizing you find her sexually attractive. Sometimes it's better to make her wonder, rather than trying to convince her. The goal is to keep her idea of "you" in her mind as long as possible, and as separate as possible from all the other clowns out there.
As soon as she feels any overt sexual desire from you, she'll stick you in that category in her mind with all the other guys that want to fukk her.
If you see her once every couple of days, a few honest, non sexual hit and run compliments can work wonders. Don't stick around waiting for her to say "thanks" or reciprocate.