I have Dimebag's book 'Riffer Madness' and he does that exercise to warm up before he plays. He starts on the first fret 6th string and hammers on - pulls off a couple of times then goes across the strings then comes back up the strings on the 2nd n 3rd fret n so on up the neck then the same with each finger till it burns.

I do it now I only go as far as the 12th fret at the moment coz I don't play that much and don't wanna tire my fingers. It's improved my finger strength and has speeded up my hammer-ons and pull-offs.