I wanted to do a practice run with sketching M-57 (The Ring Neb), so I asked my daughter for some tips as she is good with art (don't know why I didn't think to ask her before) and she has given some really good tips...
* She demonstrated that an HB pencil is better than a 2B pencil in that it smudges. The 2B wouldn't smudge. Your thoughts Alex?
* Draw the ring starting from the outside and work towards the inside to produce a better effect for smudging. Wet your finger slightly and smudge. Wow.
* She also said with using white paint, that you can smudge it for shading by adding some water.
* You can do stars with white paint using a fine tip paint brush (I had a go and is quite effective.
And a Question.. 
From what I understand, you do the sketches on white paper, and then you use black paper to do the file transfer. I don't understand how you get the whites of the stars on the black paper - do you re-do the stars with paint?
By the way, I don't know how people sketch in winter (I can barely feel my fingers, let alone hold a pencil! Sketching for me is purely a warmer weather sport.