Having recently encountered On The Practice and Science of Drawing (now one of my absolute favorite books,) I've been inspired to revisit other resources in light of what I've just read. In this pursuit I found myself observing the work of Glenn Vilpuu as he expounded on those ovoid and spherical forms so characteristic of his drawings.
As is so often the case when revisiting topics you supposedly already know well, I was suddenly struck with realization of why he favored these shapes. The sphere could not lend itself more perfectly to a sense of symmetry, while simultaneously being perfectly simple to sketch.
This impressed upon me to do some sketching with particular mind to the symmetry of primary shapes I use in my drawings, rather than leaving symmetry as something to be later considered in a refining stage. It is an excellent way to focus on the voluminous nature of your drawing. By constructing shapes with particular regard to parallel surfaces (given that all simple forms must have these) before perpendicular ones, you will find yourself more thoughtful of volume thoughout your drawing process.