I'd like to know more about how long you've been doing resistance training for, and what equipment you have access to. If you've been training for a year or more, you should be able to safely go into max strength training, so, furthering on from the advice above, you may be able to do sets of 8 for a few weeks, then sets of 6 for a couple more weeks, then 4, 3, 2, 1.
If you have access to both an assist machine (which you load up to take weight away from your total body weight as you do chins or dips) and cable lat pulldown, I'd recommend doing strength sets on the assist machine and then speed sets on the cable machine (eg 3x5 with good technique, at a rapid but controlled speed, at a weight that you perhaps could have done 10-15 reps of per set). Having both of these in your program (perhaps on alternate days) should help you build the strength required to perform bodyweight pullups.