How to work on my abdominals.

I want to exercise my abdominal to tighten my abs and reduce my waist but I don't need to tone up anywhere else. I'm wary of loosing weight because I don't want to loose my bust. My stomack is already pretty flat when I tense, but I'd like it to be more like this link rather than this when I tense link and this when I don't link . I heard that just doing crunches and sits up actually make a girl lose her waist and become too masculine. What should I do?
How to work on my abdominals.
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