Taken from random googleage..
'negatives
You are much stronger on the negative portion of the pull up. In other words, you can lower much more weight than you can lift. Just because you can't pull yourself up doesn't mean you can't lower yourself.
Put a bench or stool underneath you and kick yourself up with your feet and then lower yourself slowly to a count of four or five. Continue until you reach negative failure (you can no longer lower yourself under control). If you have a training partner, your partner can help you up.
Suppose all you can do is four reps. Your progress might look something like this:
Workout 1: 4 reps, 4 negatives
Workout 2: 5 reps, 4 negatives
Workout 3: 6 reps, 3-4 negatives
Workout 4: 7 reps, 2-3 negatives
Workout 5: 8 reps, 1-2 negative
Workout 6: 9 reps, 1 negative
Workout 7: 10 reps
Workout 8: 11 reps
Workout 9: 12 reps.
I've seen many people go from ZERO reps to sets of ten or twelve in a matter of weeks using this technique.'