On top of what Jack says, as i like to say to people...
Contrast is your friend. Make them shadows and highlights count, else they might as well not be there. :v (although obviously the highlights on clay are going to be less shiny than those on chrome)
It's harder to spot mistakes with faint shadows, and i know that the smooth look that it can give is tempting (i've been there), but do note that reality actually tends to have rather significant jumps between light and shadow, so it's much easier to underdo than to overdo. Hell, you can get away with having the darkest shadows be as much as 75% darker than the midtone! (
cylindrical case in point; midtone is 60% brightness, darkest shadow is 15%, middle shadow is 30%)
Sometimes you can even get away with having completely black shadows, but that's usually reserved for certain stylizations. Do note that if you use more than one colour for the shadows, you should rarely jump directly from midtone to darkest shadow.
this eye is TOO WELL MADE AND TOO AWESOME
allow me to show you how to make a proper badly made eye
-snipsnopsnoodle-
Wait, so, like this?
eye see what you did thereoh yeah also i was doodling in Krita again and lo and behold my immense creativity takes an unprecedented form
I was dumb enough to not place the construction lines on a separate layer, and i couldn't be assed to remove them by hand. :v