Stiffness in drawing (especially girl drawing) is commonly linked to right angles. These are most noticeable and least forgiveable when the figure is seen in profile or semi-profile.
A - In addition to being completely rigid, this is a pure profile. It resembles a diagram. Dull!
B - The posture is relaxed some, and by use of overlaps we've moved into the realm of semi-profile, but there's a missing element : glamor.
C - The 90 degree angles are stretched outward even further, and the result is glamorous, even in semi-profile. A similarly eye catching result could be obtained by making the limbs and trunk compress inward. In either case, the 90 degree angles are vanquished.
It's not always possible (or desirable) to avoid right angles, but in such cases it does no harm for them to be rotated off-kilter so as not to be perfectly squared with the canvas.
Try doing a "right angle check" on your pinups-in-progress and see if "fixing" them doesn't create a much more natural look.