Observance:

   I know I've gone over this before but here I go again.  It's this simple: you can only draw what you know.  You must observe your subject.  The more you observe, the better you'll get.
   Do the same when you are drawing from a photo.  The number one culprit for poor drawings in my opinion is a lack of attention on the artist's part.  Observe every single detail of your subject.  From that tiny hidden mole next to their nose, to the funny crook in their eyebrow, to the cupid's bow on their lips.  Notice how the shadow's and highlights define a person's face.
  Sit up, pay attention; watch that your pencil doesn't continue moving without your thoughts leading it.  You'll see an amazing progress in your art.  Believe me.
 

References:

   Other artist's examples have been so important in the past to my improving.  Studying someone's work allows you to to understand their techniques.
   I've just lately been learning more about the human anatomy, and you would not believe how quickly I learned after looking at good artist's depiction's of the body.
   There are tons of excellent artists out there who know a lot, and there is no reason you shouldn't absorb their knowledge.  They've done some hard work, and you can learn from their work and what they've discovered.  Pretty nifty.
 

Patience:

   Though this is a lesson you may not take to heart until you learn it yourself,  I can't stress enough how important patience is to your work.  In the past when I was working on a drawing, I wanted to see the finished product so bad, that I rushed through to get there.
   But the truth is, your finished product merely reflects the effort made before it.  The work behind a patient, carefully planned drawing will show; and the lack of work behind a rushed, unobservant drawing will, well....show as well.
   Being an artist is all about loving the feeling of creating something from yourself; so enjoy the process, take your time, and watch your creation unfold.
 
   

At Its Best (The Mask):

   Don't let yourself be intimidated by a finished and perfected piece of art.  Sometimes it's hard to see all the steps that came before it, and you start to feel like you couldn't even dream of accomplishing it.  But believe me:  All art starts out looking pretty crude, and it was only through a lot of steps that it came to appear the way it does.  It just appears that way.
   It's like a mask.  It hides the story of all the crude misplaced lines, smudges and whatever else that took place to get to where it was.  I know very few artists that can whip out a perfect picture just like that....besides, that is....Disney animators ;0)
 

Books:

   Art books can be an absolutely tremendous help in learning.  Go to your local art store and check some out.  The library should have some also.  There is a artist's book club called North Light, which has some awesome tutorial books and good deals.

  Visit them at the Northlight Books website to learn more.  Subscriptions are free.

   The one book I recommend you read is How to Draw Lifelike Portraits from Photographs by Lee Hammond.  It is invaluable.  It'll cover everything I can't even hope to include in this guide.