Also, if your face is going to be animatable (open the mouth, etc.) you'd also want to add an opening here. For the sake of the tutorial, I'll go ahead and do this as well.
Okay, if you didn't add detail to the lips, you'll need to add a little to the center for this. Select the innermost set of faces on the lips and click the Inset button. Now since we're using symmetry, it's going to create some extra faces in the middle but we'll deal with that. Adjust the inset slider so that it's about halfway into the faces you selected. Click Okay
Select all of the inner faces including the extra middle ones it made and Delete them
Select the verties on the top and bottom of the opening that aren't centered and move them to the center. (the left image) Next go to edge mode and select one of the edges of the new inner mouth. Click the loop button and it should select the entire inner edge.
When you have an edge selected and hold down SHIFT, you can move/rotate/scale that edge and instead of moving the selected edge, it'll actually create a new edge and do the adjustment to it. So with this ring of edges selected, hold down the shift key and move them back, into the head. It'll create a new set of faces going back into the mouth.
Pull it back, scale these edges taller, hold shift and pull back another row, etc. Do this several times. If it makes it easier, turn off the "show end result" toggle in the modifier stack. This will remove half of the head from view and you can look at a cross section of the head and plan the mouth cavity better
Don't forget to widen it but make sure that the inner mouth cavity never goes outside of the face. Also, make sure that the center of the mouth stays along the center of the rest of the head. If any of the end verties aren't along the center of the head, there will be a hold in the head when symmetry is turned back on
Smoothing Groups - Q.) Why do some parts of my face look all flat and faceted, while othe parts looks smoothed over? All the new stuff I add from inset and from creating new faces all look all flat. what's up with that? A.) The answer is smoothing groups! When doing high-poly modeling, you rarely bother with smoothing groups, but when you're doing lower-poly modeling, it's a nessecity! Go into Poly select mode and select your Entire Face. Now scroll way towards the bottom in the editable poly parameters area and find the thing that looks like the image below. Click on one of the numbers (any one, I usually start from 1 and work my way down as I add more groups). So click on 1. Now all of the faces are smoothed together. They're all in group 1
One thing I like to do is give the bottom of the upper lip it's own smooth group so deselect everything and just select the upper lip area. Now in the smoothing groups area, unclick #1 and click #2. Now there is a hard edge along the outer part of the lip, but the lip itself is smoothed. Smoothing groups can be very useful in lower poly modeling. It can create edges and give better definition to areas where there is little poly-information
Next we'll move onto the nose. A nose can be difficult to get the correct shape with, so it's not something you should just breeze through. Don't worry if you don't get it right the first few tries.
We'll have to cut in some edges, and probably extrude a couple faces to get the right shape here. Forst lets make sure that the existing geometry is shaped good so we can get started without too much trouble