Yeah this is a bit embarrassing.  I was cleaning through one of my older folders and I found this old sketching exercise that I did back in 2009 (Image 1)

 Rules:

• 15 min
• not allowed to use erasing
• no references allowed
• Monochromatic
• Opacity on brush allowed

Image 1(old)

I think with the first one I used a 50% opacity on my round brush.  So to compare to how I am now, I redid the exercise to compare. (Below: Image 2)

I also used a 50% opacity and  tried to keep it as close as I can with each other.  I didn't use up all 15mins and did some tweaking to the angle and kept to a lighter shade.  Still lots of room for more improvements!  

I think revisiting old sketches are always fun and you can really feel the difference not only with technique but also the familiarity you have with your working tools.  =)