In order to force improvement upon myself – kicking and screaming if necessary – I try to make a point to do a daily practice sketch. What I do is this:
1) Find a nice figure drawing. I like The Satorialist for this, and DeviantArt has some awesome stock art photographers for this purpose. I make the image big on my monitor, clear away my keyboard, get out some printer paper, and then try to do a loose but accurate figure sketch.
2) When the sketch is complete as I'm going to get it, I'll scan that thing and bring it up in Photoshop, then open a copy of the original photo. I drag the sketch over the image, resize it, then adjust