The book is DRAWING ON THE RIGHT SIDE OF THE BRAIN and its got some pretty interesting exercises for drawing what you really see as opposed to what you think you see. Definately check it out.

Basically I think what Mirek is trying to say about neg vs pos space can be summed up in one word: Silhoutte. Actually it is a very good practice to render (or adust your render in PS) a character where they are displayed as a solid silhoutte. Its an awesome way to judge proportions and center of gravity and to see if you're on the right track of making a character as instantly recognizable as possible to an audience.

I would say that the more complex shape of the helmet and head is making the rest of the model look way too simple. What stands out to me as needing the most attention is the shoulder piece.

Nice job so far, dude!

Dave