Normally, the first thing released for a custom "BodyShape" is the two-part nude mesh, which can be linked to a skintone (Maxis or custom) to make a "shower proof skintone." Shower proof skintones allow your Sim to retain his or her custom shape when true nude, such as in the tub or shower or when getting out of bed after woohoo.
Other useful things for a BodyShape are Poser magnets, which can be applied to any mesh using Poser to adapt the mesh to look like the new BodyShape, a full body nude mesh, and nude textures for all 3 nude meshes (top, bottom, body) also known as nude clothing.
TheTigerKing, in collusion with Chris Hatch from Back Alley Sims, has created the "BodyShape Organization Kit," which helps players add custom icons to their custom sized clothing to make it easier to dress your Sims in clothes with similar appearance to the nude meshes.
If you'd like to play around with a BodyShape, I recommend trying one of Warlokk's. I have a tutorial here: http://www.woodensimolean.com/node/251 I might go so far as to set up a new set of game folders so the BodyShape stuff isn't accidentally mixed into one of your favorite hoods.