While I've been on the hunt for a cast of a bust to draw from in person, I've been doing studies of marble busts from photos taken by the MET and put up on their online archive. It's been a great boon since the lighting in them is very good and accentuates the forms of the statues incredibly well.
Right now I'm finding I have trouble depicting the soft forms of female heads, and I suspect most of it has to do with my edge control - Jeff Watts pointed out that I'm leaning towards mostly hard edges for my mapping and all that, so I need to become much more conscious of my edge variance and work more soft, firm, and lost edges into it to help bring the drawings to the next level. So that's my goal for the time being!