Sometimes it is good to practice things that are most difficult to paint. And every painter has something that is a challenge. For me it is flowers. They are really difficult to paint without them appearing flat.
One way I have learned to practice to try to improve my technique is through tonal studies. I paint the set-up in tones of black and white, usually with acrylic paint. Then I allow that to dry.
Next I add some color, using oil color and a glazing medium, I paint thin layers on top of the tonal study.
I continue to adjust the colors until I get it to a place I like. Often, once it is at this point, I will set it aside and look at it for several days, deciding what else it needs to complete the painting.
And that is where I am with this study. I continue to tweak it. And maybe one day, when it is really finished, I will post another photo of it.
The Studio 
Just keeps getting better and better.