I participated in a Blood Princes kill with my guild tonight and thought I'd make a warlock/caster guide for tanking Keleseth as I was unable to find any when I went looking before I was supposed to tank it. I'm happy to answer and questions or respond to any suggestions. Hope it's helpful!
EDIT: Apparently there was a recent hotfix so that Keleseth will no longer melee you. Still be wary of his position, however.
EDIT: I've had several questions asking why I didn't choose a spec deeper into Demonology for things like Master Demonologist. My original goal was to take a spec that was already a common one for raid DPS, and just tweak it a little to make tanking a little easier. This way people that may not have dual spec will have a spec they can do moderate DPS with, but still have some helpful tricks for tanking. Quite frankly, it's completely doable on normal mode without any special spec anyway. :)
EDIT: Many have commented that I'm not using an armor type. I am, actually, but for a cleaner UI I used a feature called 'consolidate buffs' that groups together certain long-duration buffs, which is why you don't see it.