As a random thought exercise, it occurs to me that Libris Mortis' True Necromancer PrC would be less awful (note I don't say "great") if two conditions were met: 1) You went into it without the intent to finish it, at least pre-epic level, and 2) you used a PrC with its own progression to fulfill either the arcane or divine casting requirements. I've no doubt someone else has had a similar conclusion before. Doing it this way means you can aim for specific gains from the True Necromancer, and you won't need to worry about the dual lost progression, because the caster level increases for either the arcane or divine sides will simply increase numeric effects, without needing to sink an extra Practiced Caster feat into keeping spell levels relevant.
With this goal in mind, which PrCs would make the best choices for leading into True Necromancer, and which break points would you aim for?