i'm not a professional modeller but wanted to share a couple of tips while doodling in zbrush.
Here's a before and after:
in the after i brought in the character design art work by Alan Stewart (http://alanstewart79.blogspot.com/2012/04/) into zbrush and used his drawing as a guide.
because the reference drawing wasn't directly a front or side view; in zbrush i rotated the cam a little to match the concept art and saved a custom view using document>zapp link properties. i could then switch between showing spotlight drawing and viewing through custom view.
hope you find this helpful.
Happy Sketching!
Nate
please check out Alan Stewart's amazing artwork at (alanstewart79 dot blogspot dot com)
also check out Dylan Ekren's 'Working with Reference images in ZBrush' for a great tutorial on how to bring in reference image into zbrush