I remember one my art teacher asked us to do once - and that was to hang a shirt on the back of a chair and sketch it.  The way the folds of the shirt work with the light was a very useful study, and getting the precision of a chair's legs / perspective is also a useful challenge.  Give it a go and see how you do.  

Also - take aaaages over sketching - be a perfectionist.  I have a sketch (digital) of Scarlet Johanson in my back-catalogue on RR and i can remember that it took at least 8 hours of (essentially) trial and error:

http://www.renderosity.com/mod/gallery/index.php?image_id=1756079&user_id=409068&page=15&member&np

Then it gives you a "i can do that" attitude and then nothing is impossible...

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography