Creating good geometry is one of the foundations of 3d. Without good geometry the final result will be poor, no matter the effort on the texture or attention to lighting. So spending a good amount of time obsessing with finished geometry can hep you dearly when you need to sort out poor geometry that has been supplied to you… or or even the unexpected results from boole operations!
I have found that learning W and F menus are invaluable when handling the small geometry detail work – as is being able to see the best way that polies can be broke down. Getting stuck in to re-make parts of the geometry by hand has been the best thing to get good results at the rendering stage if the geometry is compleately knackerd from a boole ooeration.
After I got used to this clean up opperation approach to geometry I had another look at retopo and found that I had a better understanding of what needed to be done – and certainly a better idea of how I could get there. Shrink wrap, and mirror modifyers help speed up the retopo process but again it is about getting your hands dirty and editing the vertex points one by one to create the contours of the model so modifyers work as they should and light reflects as it should… also useing the crease weight markers in the T bar along with bevel set to weight helps when then adding a sub surface.
Unfortunatly when it comes to problem solving the compression of quad numbers down effecivly i have no tips or advice… im still running on trial and error… and i need to solve the pinching that appears from this….
Hope this helps you approch retopo as a subject… I have found it invaluble to learn as my understanding of geometry has grown just by the practice