I've started a new 100-page sketchbook that I'm filling with just portraits. Specifically, 3 portraits per page, totaling 300 when I'm completely finished. It's mainly for keeping sharp with my draftsmanship skills, so I'm trying to keep each head to around 20 minutes. The early ones used a lot of shading, but I'm now standardizing the style to be more line-based, with a head-on perspective.
28 pages into my "300 Heads" portrait sketchbook.
11/12/12 Update: I've started using #300Heads as the hashtag for my Twitter updates regarding the new portraits.