Watch the Hammer/Pull-Off online guitar lesson by Jeff Scheetz from Rock Tricks
The idea behind hammer-ons and pull-offs is that your left hand can probably play faster than your right hand can accurately pick. So, we just use that to our advantage and let that left hand go to town! As with any speedy technique, it is really important to start very slowly at first and gradually work up speed.
This is especially important with hammer-ons and pull-offs because it is hard to get all of your fingers to play at exactly the same speed. Play with a light touch and on your finger tips for a clean sound.