This is such a terrific drawing exercise. It seems really dumb, but if you are having a day when everything you draw is AWFUL, you need to do this exercise. It will get you back on track.
This is from New Drawing on the Right Side of the Brain Workbook by the amazing Betty Edwards. For some reason this book is not available at Betty Edwards' website, though you can buy a large kit of all her excellent materials.
You do the exercise by putting your drawing pad almost out of sight, but just where you can still draw with one hand on it easily. You take your opposite hand and hold it palm up. The idea is to never look at the paper while you are drawing. And then you just draw all the details — wrinkles, scars, whatever — on about a one inch square section of your palm.
Your drawing will come out looking something vaguely like this:
Looks awful, right? Doesn't matter. How it looks is not the point. Apparently this exercise lets your rational, language-oriented side of your brain take a break, and the more visual-oriented side of your brain to take over.
You can do one of these contour exercises in just five minutes, and if you're feeling impatient, even three minutes might do the trick. Then take on that drawing subject that was giving you such a headache. It may suddenly fall together.