Although I understand the concept of blend shaping, as well as the procedure, I have actually struggled with this quite majorly. Although it is something that has a straightforward method, it can be very hard to locate and rectify errors when they arise. I pretty much learnt how to do blend shaping properly through trial and error, perhaps the most frustrating style of learning.
One error that kept coming up was that when I produced variations of the Emperor model to be part of a blend shape, instead of the original model staying in its place, it would move into the same position as the blend shape. In addition to this, it would also make certain controls unmovable when a blend shape is turned on. I learnt that the way around this was that prior to beginning the process of blend shaping and indeed skinning, all transformations must be frozen and ALL history should be deleted so that there are no odd inputs and outputs to the model that are disrupting the blend shaping. Freezing transformation and deleting history are two fundamental elements of modelling that I have discovered that far too many modellers seem to forget about, leaving us riggers to clean it up.
Another problem that arose frequently was that when I tried setting up blend shapes at times, it was not possible under certain settings to have two blend shapes switched on at the same time. I.e. if a blend shape to make the mouth wider was switched on then it would not be possible to simultaneously raise or lower an eyebrow. I resolved this by resetting the Blend Shape settings to Front of Chain and then began making the blend shapes from scratch.
As well as learning from my errors, I have also learnt how I can have numerous expressions (i.e. variations of a model that will influence how the original will alter when the blend shape is switched on) assigned to one blend shape, as opposed to having loads of blend shapes for every tiny action.
This can be done two ways, either by opening the Blend Shape window and creating a blend shape. For each expression (or part of an expression) select the original model, click "Add Base" to create a copy of the model and then alter that model to convey the expression.
The other way is to create the model from scratch and then to go to the Add Blend Shape Target Window, in which I then shift select the target model, followed by the original model and then select the relevant Blend Shape from the Existing Nodes menu. Either method will work fine.