A master copy is where you replicate a classical painting, thereby reverse engineering the techniques that they used to achieve a successful piece. If you're doing this digitally, like I did, here are a few ground rules.
1. NO tracing!
Hone your artist's eye for proportions by using a grid. Using this method also forces you to pay attention to the volume of objects in the image, rather than simply tracing the lines in a mechanical fashion. One way to do this is to set up a grid using Guides in Photoshop.
2. NO color fills!
Paint in the gradient of the first layer with brush strokes instead. Color fills just make the image look mechanical and plastic if you paint because the gradient is too perfect.
3. NO color picker!
Learn to eyeball color instead of using the color picker to pick them from the original. This is to force you to guess how it was mixed and be mindful of layering, as it's important to digital as well as traditional painting.
Here is my master copy as an example. I've blogged about what I learned here, if you're curious for a more in-depth look.