1. Don't have your subject too small in the photo. The bigger, the better, the more detail can be achieved in the drawing.
2. If you have a number of photos, then that's good. I can still do a drawing from one, but a couple of photos is always better than one.
3. Close up shots of the face or subject are very good for portraits and drawings. Keep it in focus though!!!
4. Use outside natural lighting if you can. If not, then try not to make the photo too dark. You still want the shading and colour to be correct and appropriate. Don't worry about 'red-eye' occuring in the photos. I can easily work around that.

Here are some examples of good and bad photos (please click to enlarge photo):

                           
Too dark and unfocussed      Subjects (planes) too far away     A bit too far away, slightly out     Great lighting and close-up
                                                                                                                  of focus, and bad shot/pose