Facial morph targets

Are there any general rules one should follow when creating morph targets for facial animation?
Thank you?

Here is what I know on this topic:

1. The model topology should be designed in a way that allows the proper facial deformation.

2. Morph tragets should work well together. For example if you create a smile and a open mouth target they should form together a smile with the mouth open not a new character from horror movies.. This is one of the most difficult tasks when creating morph targets.