The 20 Minute Rule
 

Treat your model/subject right. Pose them and paint for 20 minutes, then give them and yourself a break - let them get up and stretch - you step back from your work and evaluate your progress.

Ultimately, you will get more time with the model because they won't be quite so cramped up and you get breaks from the work to evaluate it.

While at a class with Bill Whitaker at BYU, Bill used this 20 minute rule to good effect with his model.