Thrall of Orcus is a pretty terrible prestige class, simply because it doesn't give you very much casting progression. There are very, very few abilities that a class could give you that make it worth giving up a caster level.
Advice for evil clerics? I'd look into Libris Mortis for items that help your rebuking level and reduce Turning Resistance. I think there's a rod and a lyre. Any undead you run into are suddenly your friends. Also, consider that the Corpse and Bone creature templates are much more powerful than the zombie and skeleton templates respectively, and you can apply these templates with the Create Undead spell, which might be a little costly, but will basically create full-strength minions for you.