I have been working out on a pull-up machine that has several different settings to lower the amount of weight I'm pulling up. Since I weigh almost 200 pounds, doing many pull-ups without any assistance at all is difficult for me. With the setting removing around 70 lbs, so that I'm pulling up around 130 pounds, I can do four sets of 10-12, but pulling my entire body weight up, I can only do 2-3 pull-ups (four sets). So what's the best way to be able to increase my strength level and do more of the pull-ups without assistance from the machine?