Check this site out - the "classroom" threads will be of real interest to you
http://wetcanvas.com/forums/channels...&channel_id=25
And for pure drawing with all kinds of specific tips and exercises - the Basic 101 series of threads is a good one
http://wetcanvas.com/forums/forumdisplay.php?f=227
The key I notice here is that you need to pay more attention to the actual shapes you are looking at instead of what is in your minds eye - a specific would be the line ofthe lip - you have drawn it going up to the right and it curves down, which changes the entire flow of the mouth/teeth/nose. Learn by doing - that is what you are doing here and it is still the best for drawing skill.