One of the most important factors, I've found, is the difference between what you CAN print and what looks best: often, you don't want everything that's on the negative. A 'perfect' negative will often look a lot better after dodging and burning to remove (for example) shadow detail in areas where you don't want it, or to darken a Zone VII to Zone VI.

Often, I find it quite hard to manipulate aggressively enough because I know what was 'really there'.

Cheers,

Roger (www.rogerandfrances.com)