NY Mom,
I definitely don’t think that saying “Now THERE’s a tsniusdig outfit” is the right approach. That’s not a compliment – you are calling attention to her wrongdoings.
How about being much more simple: When you see her dressed b’tsnius, say “You look nice today” and no more. Never call attention to the mistakes.
People usually know when they’re doing right and when they’re doing wrong. If you compliment her appearance when she is tsnius, and say nothing at any other time, she will understand on her own what you are appreciating (on a conscious and/or subconscious level). Don’t be surprised if after a while she starts to walk straight over to you every time she is wearing something tsnius, just to get your approval. That’s how powerful this method is.