Here is some of my work today on Gorilla mouth deformation. (original design and model).
I like to do a blocking pass (like gesture drawing on pencil and paper) (1hr) and a polish pass (2hr -- this is the more technical version, sometimes i found it helpful to just grow selection of challenge area then unhold influences associated to selected vertices (can use component editor to find these influences) then with a soft brush and small add value try to fix things. For major challenge area i liked to use Michael Comet's skin weight tool to fix most challenging vertices first to make it easier to paint ).
Here's the blocking pass:
and here are some pictures of my enveloping:
Michael Comet (comet-cartoons dot com)