1. Don't strive to draw photographically...strive to draw artistically. Look at drawings you like (Michaelangelo, Degas, to name a few) and try to use some of the technique the artist you admire did. If you like red chalk drawings by Waterhouse, try doing a piece in red chalk. Take what you like from any artist and put it into your work.
2. Choose a good photo. Make sure there is a sense of light and shadow. It is hard to work from "studio" photos and have them look natural (especially very posed pictures of children). Bad lighting on a photo will usually equal a bad drawing.
3. Take time to analyze the pose of the person in the photo. Don't just copy the 2 dimensional. Think about how that is a 3 dimensional head in space. Imagine you are sculpting that head. Try to think about the forms as if you could touch them.
4. Allow yourself to warm up before jumping right in to working on a long piece from a photo. Do some "gesture drawings" from the photo. Try to get the essence of the pose and the emotion. Make some thumbnails to show composition. When you are ready to work on the final piece...PLAN YOUR DRAWING. There is nothing worse than starting from the eyeball and realizing you can't fit the head on the page! Start with a plan.
That's what I have so far..any advice welcome...