Let me suggest a few different ways, supposing that you can at least do 1-2 full pull-ups:
1. Take as much time as you'd like in between sets. Say it totally tires out your muscles after a set of 2. Take 10, 20, or 30 minutes, and then come back to do another set. You can even take a half day off and do another set. This does not get you any kind of cardio workout, but it will train your arms, shoulders and back muscles to help you do more in a row.
2. Change grips. For most people, wide-grip pullups are harder to do than narrow-grip pullups. Also, change the muscles you use. If you turn your palms inward, then you use more of your biceps. A lot of people find this easier than the regular pullups that stress your triceps. Switch your grips for each set, and you should be able to squeeze in some more.
3. Use momentum and cheat: Let your body swing and you will be able to double or triple the number of pullups you can do. Some people regard this as cheating and not worth doing. But this certainly has some real-world applications. For example, let's say you need to climb up over a high fence for some reason. The dumbest thing you can do is to minimize your body momentum and try to pull yourself up just by strength of a few muscle groups. In real-life and sports, you need to learn how to use your body's momentum, so as to move more effortlessly. Anyway, I digress....
4. Lower your bar: If you have a door-post bar, lower it after you've tired yourself out. You want to lower it to the point where you can touch the bar at around your face- or chest-level. Now, stretch your legs out forward, and let your feet touch the ground. This should take some of your weight off. Let your knee extend all the way, so that your legs won't be helping you. You just want to take some weight off, that's all. Now you will be able to do a few more.
5. Squeeze your grips and buttocks hard. This is a tip I got from the Russian fitness guy Pavel Tsatsouline. If you pre-squeeze those body parts, you should be able to do the pullups and pushups more easily, he says. I think it works for some people.
Good luck.