In reply to Kemics:
I don't know how useful it is for improving lock off strength, as you kind of need to be able to lock off in the first place to perform the exercises, but that CWP fingerboard program worked fantastically for my grip strength, I went from being able to one-hand dead hang a standard campus rung for less than 1 second to being able to hang for 10 seconds in any arm postion, all in 6 weeks!
If I hadn't popped a finger pulley (in an unrelated incident) I'd have started another cycle, i'd be doing one arm chin ups on doorframes in no time!
Has anyone suggested trying to do lock offs in order to improve your lock offs? Sounds obvious but it seems the most straightforward route, find two holds, lock off with one arm and use minimum pressure with the other hand (or use a much lower hold) in order to hold the lock.