I made my first pieces of fitted mesh today.  Messing with the avatar shape sliders points out that several of the bone changes influence the appearance of the mesh in ways I didn't expect, especially around the hips and butt.

 

I used Blender 2.7, the fitted mesh survival kit, and a shirt from Damien Fate's free clothing mesh.

The most important tidbits?

• When weighing, four or fewer bones influencing each vertex. This means you can't just copy the weights from the avatar models. The area around the shoulders has a LOT of influences.
• If you aren't using some of the collision bones, you can delete their vertex groups and the mesh will still upload into SL. You cannot do that with the normal bones.
• Don't have the joints box in the SL viewer uploader checked.