Prompting for "slim waist", "narrow waist", etc. will only take you this far.
This Lora will take it farther, much farther... until you'll beg it to stop.

Does a pretty good job of maintaining "natural" curves and leaving the rest of the image intact.

Somewhere between strength 0.5-1.0 the results become anatomically implausible. By 2.0 you're deep into Toontown. Also, at higher strengths exposed midriffs will become toned (not jacked) and plasticky - nature of the dataset.

Negative strength adds chub, not flub - Might be useful in it's own way!