Once you've attached the mouth to your head, go ahead and finish off your loops. Finish off the back of the head and neck. Now is the time to add in any little details that you want. When you are finished with the one half of your model, simply duplicate it, combine it and merge the border edges the same way you merged the mouth to the head. Try smoothing it when you are done to see how it looks. Go to polygons>smooth.
Looks like this guy only needs one more thing to be complete... Wrinkles. Right now he looks a little smooth.
To create wrinkles while maintaining perfect "quadness" is a cool little trick.
Find the edge at which you wish to create a wrinkle.
Along that edge draw another edge just a little offset to it and connect at the corners.
Next divide the faces in the center, leaving the triangles at the ends alone. This will make quads out of everything.
Then pull the center edge inward to create your wrinkle.
Try smoothing your object to see the result.
Here is the version where I added in some wrinkles.
And here is the final version after all the detail has been added.
Happy Modeling :)