I've also been collecting reference pictures to practice sketching. Here are some of my favorites.
This silverpoint drawing by Leonardo da Vinci is possibly the most beautiful drawing of a woman's head ever. (from WikiArt.org)
Another Leonardo head, this time in charcoal and chalk. (from The Metropolitan Museum of Art.)
I really love these sketches by John Singer Sargent.
They were studies for this painting. I read once that he worked on it for a few minutes every day. He would play croquet with the girls and their family in the late afternoon, but as soon as the early evening light was just right he would hurry to to the painting. (from The Tate Museum).
I'm having trouble with my camera. When I (or, more likely, my older son) solves the problem I'll share some of my own sketches. I'm trying to draw heads, faces and figures every evening not long before going to bed because I heard that makes you learn more quickly--something about the way the brain consolidates information while sleeping.