The vertex weights of the default character mesh could be better.

There are a few things about how the default Second Life avatar mesh bends with animations that has always bugged me.
The first it the fold that appears in the chest with some minor movement of the upper torso.
The second is how the buttocks deform when sitting down or bending over.

I've spent some time working over the data files used by the Second Life client and have attached my modified files.

I've only changed the vertex weights in the upper and lower body meshes. But have included all the meshes that include vertex weight data because I've resorted the vertexes by vertex weight. I did this to minimise the number of times the viewer needs to calculate a new transformation matrix.

In both the upper and lower body meshes I've fixed some asymmetry to the vertex weights. In all such cases one of the vertices was associated with one bone and the other with two. And I've mirrored the value from the vertex that was associated with two bones. This include all the vertices in one ankle.

For the chest fold I just smoothed out the transition in the front of the avatar between the bones labelled chest and torso.

In the lower body I've smoothed out the transition between the pelvis and the upper legs at the front and between the legs. And at the back I've spread the transition area upwards so less vertices on the avatars behind only move the the pelvis.

I'm sure someone who knows more about weight painting could do a better job. However most of my time working on the issue has been writing custom file conversion tools.

I hope me putting this here is of help to someone.