It's easier to think of them as shapes, not what they are. Once you simplify them, it becomes easier. I have problems with the colours to use when shading :/ I'm getting better, but my skill level lays in grey scale still ^^;
Well, if you know how to shade a sphere, a cylinder, and a box, if you break all your objects into simple planes that mimick them, then it's easier. Also drawing from life helps you visualise things in 3D easier. I found alot of things clicked for me after I did two terms of life drawing.