Using a Motif

                                                              Kirkwood Market II

One of the best ways I've found to stretch my limits in painting is to use the same subject again and again, but paint it as differently as possible each time.  The first time you might use local color.  But the next time try using only three colors.  The next time eliminate everything in the background.  The following time raise the horizon line; the next, lower it.  Paint wet into wet, but switch to painting on dry paper for the next painting.
Bring the subject up close, then push it back.  Paint the background to leave a white silhouette (painting the negative shape); then paint the object in a darker, positive silhouette.  Paint in neutral tones, then in pure tones. 

Experimenting will give you  options.  You don't have to follow the same method every time.  Deciding what you want to say will influence the procedure you choose.

Here's another take on the Kirkwood Market with a cooler palette and more pure colors.