For a humanoid model there should already be vertices at or near the centerline. Just tab into edit mode, select all the vertices on the half you don’t want (excluding the ones ‘on’ the centerline) and X --> Delete Vertices.
Now select all the vertices that are on the centerline and press S (scale) and X (presuming that the YZ plane is the mirror) and scale to zero. (This makes them exactly centered.)
Now Tab back into object mode and press Shift-D to duplicate the object and again scale (S, X), but this time by -1.
You may have to position the mesh (using G [grab]) if its centerpoint is not aligned to the center of the complete humanoid).
Shift-Select the original object and press Ctrl-J --> Join Meshes.
Tab into edit mode. Press W --> Remove Doubles.
That should do it…
Hope this helps.
Man! I’m always one click behind someone… :< [/edit]