Well, as I've learned, you have to model the mesh as much in quards as possible, because 1. the mesh will deform better, and 2. it'll give you less stretching of the texturs.

Also, the wire needs to flow according to the muscles in the face, so that you'll get alot better deformation when you animate it.

This illustration will hopefully make you understand what I mean (Think I got this from MattTheMan).

It's abit hard at first (atleast I think) , but it's really important if you wanna make it a solid model that is good for animation.