When we look at an image of a pretty girl there are many things we may respond positively to, though one of the first things (and something we should make good use of in our pinups) is the overall stamp of what we're seeing and the information it gives us.
Knowing when and where to supply this information is part of the fun of picture making, and should be left to your own judgement, but there are some titillating tricks that can be employed to enhance the sex appeal of a silhouette.
SILHOUETTES ARE THE CHEAPEST WAY TO SHOW FORM AND PREVENT YOU FROM HAVING TO DO A TON OF RENDERING TO GET YOUR MESSAGE ACROSS.
A) A sassy, but fairly standard contrapposto pose. With the arms and ribcage at this angle, the bust gets lost inside the outline.
B) By brushing back her hair, turning her ribcage slightly and losing the left arm behind the body (the hand re-emerging at the waist), we get her left breast breaking the silhouette. Since we already have her right arm outlined, this is welcome information!
C) A more drastic twist gets the face into the silhouette.
D) A full turnaround shows the curvature of the buttock and breast in the same pose. An earring has entered the silhouette here, as could any article of clothing.
E) Sitting up pose, head-on. Fetching hip and hairline, but...
F) ...a slight twist of the ribcage gets the breast into the silhouette. The bordering hair and arm line help in sending the eye here. There is also the somewhat vulgar option of showing where the buttocks meet the ground, though depending on your aim, this might be an area worth drawing attention to. Curves are exciting, and it's an area that shows contact with the ground.
G) A more dramatic twist (comboed with a 3/4 view) gets the breast and hands into the silhouette. If toes are important to you (as they are to some), they could be just as easily defined in such a pose.
Group 3 - Hair
H) By allowing eyelashes to break the silhouette we get a true sense of their length. But oh no, look at that bland, blocky hair shape! It's so uniform.
I) This is better : a tapering, non-uniform shape with lots of width variation. Even though the shape is curvaceous, it conforms to a geometric structure (and one more interesting than a parallelogram). But there still remains the option of breaking it up a bit...
J) ...like so. Note the use of "lost" and "found" hair shapes for the finer strands, giving the impression that, in parts, the hair thins out so much as to be indiscernible.
K) Breaking up hair silhouettes is not without risks. If you're going to puncture hair with holes and break up its outline with fussy "stray" strands, it's best to do this at the edges of your shape...
L)...rather than the center. See how the contrast and detail level is too much? Further, any holes that are of the same size/shape, or strands that are of the same width tend to make hair look less natural and pasta-like. Make sure you maintain a balance between big, medium and small shapes.
As a final note, you should always choose a hair shape that best suits your pose/composition. Use it as a means of leading the eye, obscuring, framing, etc. Don't let it be arbitrary; let it be a tool.