demon432's
AlignNormals and
ExtendedManualEdit are invaluable assets as well. When you add vertices, the smoothing gets blown to Hell, and these allow you the greatest flexability in correcting the smoothing issues without having to resort to MilkShape's rather lacklustre "smooth all". I honestly would not even try to add vertices without them. Get them
here.
wes_h's
UniMesh plugins are an absolute must - download the version from
here, as they are newer than those included in MilkShape and have many bugs worked out.