[llamabeast]

Well I just treat each limb as a cylinder, roughly. So the leftmost edge is light, then a line of medium pixels, then dark. Then I go over and modify bits depending on their angle (generally the upper leg is more horizontal and therefore its top surface would catch more light). Then also I darken bits near joints. e.g. the top of the inside leg will be very dark. I take quite a, erm, non-artistic approach to it - I just try to guess how much light each bit will catch.

Also more recently I have been using colour filters to alter a load at once, saving heaps of work. So if I think the lower leg should be a little darker than the upper leg, I just select the whole lower leg and decrease its brightness.