I would suggest searching google or youtube for tips and tutorials on drawing or painting realistic faces. The main things to look for are basic proportional guides that you can keep in mind. You don't need to follow any particular rules, but knowing the basic measurements of a typical head will help a lot and give you something to work from. Things like "there's one eye-width between the two eyes, and one between each eye and the outside of the head", or "upper lip is halfway between eyebrow line and tip of chin". Note that Sculptris shows the model in perspective, so proportions can be a little bit tricky to measure. It will look slightly different depending on zoom level.
Another big thing is to look at a reference photo or several, to help get an idea of what forms you need and where to put them. Resize the Sculptris window and have an image visible beside it. For making a 3D head it would be best to have a diagonal view, or ideally several angles of the same head so you can cross-reference with parts of your own model as you go.
My main sculpting tip would be to use as large a brush as you can for each shape that you're making. If you are adjusting a cheekbone or the back of the skull - use a much larger brush than if you're doing the nose or lips. Work from large to small, and make sure the large and rough shapes look good before going to details. You might want to try blurring your reference images to see if the general shapes seem right. It could help you look past the small details.
Wow, thanks a lot for the in depth introduction to faces
It seems a little complex, as I'm not an artist, but I think I can understand it eventually. I think I might try some kind of contempary style or "not-strictly-by-the-rules" style so I can model faces in my own way.