In professional productions, it’s common to create several more clusters with a blend shape deformer for finer control of facial expressions. For example, you can create clusters for a number of mouth positions that you can use to simulate the phonemes of speech.
When you used certain blend shape slider values, you may have noticed that the face looked good in all regions except for a few vertices. To fix such regions, delete the cluster, move the problem vertices of the target face to a better position, then create a blend shape for the improved target face.
As much as possible, use a target shape that’s not extremely different from the base shape. If you need to use an extremely different target shape, create a blend shape with multiple targets. Each of the multiple targets must have a shape that is progressively more like the extreme target shape. Before you create the blend shape, turn on In-Between in the Create Blend Shape options window, and select the multiple targets in this order: least extreme shape difference first, most extreme shape difference last.