If you want to develop your pull offs you'll need to develop lots of finger strength. One neat exercise involves hammer ons.. Go from the lowest string to the highest string. Just hammer on 1,2,3,4 keeping each finger in place. You'll find yourself loosing volume as your pitch increases, especially if you try this on acoustic, this allows you to gauge your progress. This kind of exercise is very tedious and is best done whilst watching tv or something unless you have a high boredom threshold like I do!
Here's another good finger strength exercise for you:

http://www.guitartricks.com/2000/trick.php?trick_id=6670

Another point, make sure the finger you're pulling off onto is close to the fret but not damping it.

I'm so bored of all this mettalic bashing that I'm posting a new thread, here's the link:

http://www.guitartricks.com/forum/showthread.php?threadid=7229