Avoid using points with more than five polylines connected to them.

I think I usually have a problem like that when there's triagles mixed in with squares.. I usually try to use loop cut ('k') when modeling instead of sub dividing.. which creates triagles if only one face is selected for sub dividing. I would redo the top corner square above the shadow into a total of 3 squares.. so you can continue your nose ridge lines all the way up to the top of the face without creating triangles. Also.. when you connect the 2 halves together.. sometimes some of the surface normals are facing the wrong way, which can also create a 'shadow' effect. When the 2 halves are together, select all the vertices and press 'ctrl n' I believe to re-align surface normals. I think this is relative the object center. Good luck. Red.

It really should be fine where all the lines come together should be fine, I do that all the time (although it could create a weird pinching effect, especially if animated--I'd merge them on the top of the head or something so it's less visible). What's happening here is the normals on one side are reversed to that of the other. You could fix that by selecting all faces and pressing ctrl-n, as reddog said.