Remember to Best Render regularly to get a clear idea of how an image is taking shape.
One thing that has saved my big butt on occasion is that even thou Zbrush is ordinarily very stable on my system everyonce in a great while I have a document get corrupted and then it is impossible to open again.
So… If I am working on something really really important I always periodically save it in two files or alternate between them. So if one bites the dust I will always have the other.
One thing that has altered my reality is realizing that ZBrush is a PAINTING application…
As a painter, I don’t want to fool with meshes, and suddenly I realized I had the ‘Dream Tool’ of a thousand painters…
No longer did I have to simulate light on tangible, now I could paint with objects, and I had my own ‘soundstage’…Place my actors, apply makeup, and ‘lights, Camera, Action’…! 
>When working on a ZBrush project remember to save the tools you’ve created along with your models, textures and texture maps.
These are not saved automatically when you save the ZBrush document.
So if you think you may need to re-use components after saving your current project document or for possible future projects… get saving.
>Think of the Diffuse and Ambient qualities of Materials as an added accentuation to lighting effects and perspective.
For example objects closer to the viewer and light source having greater contrast and/or tonal range,distant objects having less contrast and tonally lighter.
Perhaps an object near a light source could have blown out highlights or an object obscuring a light source could appear more silhouetted.
These treatments can often add a more graphic/illustrative renderings of depth outwith the use of fog or depth cue rendering.
If using fog a touch of blue in the fog always cools and knocks back any hot
hues which helps the eye percieve distance reccession.
Another tip
If you have favorite materials, number of lights, canvas size etc… that you seem to work with all the time.
Set up a document call it Blank for example
Set up canvas size, lights etc…
To save your favorite materials
import them into the Material palette and on a blank layer place a dab of each on the canvas
Save the document
Then whenever you want to start a new project using these parameters load up “Blank” and away you go.
You could have a number of these starting documents set up and save alot of time