I'm not actually MAXIMIZING effectiveness here, just throwing out some thoughts
Max out Leadership score and go hunting for a Cohort in low places... Have the Cohort be a high level caster/undead able to create spawn. Have THEM take Leadership as well (or already have it - you're just recruiting THEIR smaller armies and incorporating it into yours). Have THEIR Cohorts also be high level casters/undead capable of creating Spawn. Rinse. Repeat.
So now, you've got significant (unlimited?) numbers of Spawn plus several casters capable of animating and controlling (via Rebuke) low level undead. Throw in a couple of magic items to increase the effectiveness of Animate Dead and Rebuke...
Have your team of Cleric Cohorts/Followers use Rebuke to 'Command' groups of uncontrolled Undead. Note that A Cleric can relinquish control of Commanded undead to establish control of NEW undead. Of course, once mindless undead (being mindless) have been given a set of orders, I imagine they'd continue to follow those orders until destroyed ("Follow that Battleflag. If you encounter a living creature, destroy it").
This should work with freshly Animated Undead as well: Once given a set of orders, they ought to keep following them even after they're no longer controlled... Just won't follow new orders. For the purposes of an Army, all you need to do is maintain control of the creatures that carry the battle flags (and their back ups).
This scales rather well, I think: Just takes time and money, a number of really scary looking battleflags and a really large field to store thousands upon thousands of Skeletons staring vacantly at said flags...
A'Mal