Step-by-step: Gradually breaking the mirror down
Creating an initial sketch using values is a great way to begin your piece, but you can run the risk of introducing too much muddy black if you use it as a base unaltered. I like to play with gradient maps on my value sketches to introduce colour into the shadows, and create colour schemes I may not have thought of initially.
When tackling the broken glass of the mirror, I make use of the Lasso tool to create guides for the sharp cuts in the glass. I alternate between dark and light lines to show the catch of the light in the seams. I make sure to add a level of thickness to the glass edges, so the broken pieces can read as flipping through space.
Broken glass flying from the mirror can give some compelling action to the scene, but has the potential of distracting from the main focus of the image. I use motion blurs on some of the shards to imply a direction of force as well as to soften them up, leaving harder edges for areas closer to the focal point.