Some of the best tutorials I have come across, for various mostly SL-specific purposes.

Robyn Huffaker's Blender tutorials for sculpts

Great "troubleshooting" descriptions of the quirks in sculpts, as well as decent blender tutorials.

Gaia Clary's Machinimatrix videos on sculpting

While I am personally not all that fond of the video format, Sculpted Prims part I, II and III in particular is a good walkthrough of all you need, from a clean install to quite competent techniques and results.

Eloh Eliot's skin PSD files

A handful (two, actually) of high-quality skins, with complete, layered Photoshop files, for study, modification and general use. (Originally BSD licensed, but then apparently loosened further as many people's heads exploded over the "credit but do not use the name for endorsement"-part of that license).

Robin (Sojourner) Wood's tutorials and templates

Robin Sojourner, to use her SL name, has some tips about texturing, in particular some very nicely laid-out templates for avatar clothes, as well as an explanation for the annoying alpha halo effect you often see.

Easy fur texture

It seems the noise/motion blur trick for making hair textures is well known by now. Here's a slight expansion of the idea, to make a fur-like look.

Subsurfing in Blender

And then my own notes on using subsurf modelling for sculpting.