I am reading this really nice book right now called the Naked Warrior by this Russian guy Pavel Tsatsouline (probably wrong spelling). There is so much stuff in that book that I really cant talk about it all here, but I will give you a few tips.
1)You can never do too many pullups. The more you do the better. Make a habbit... For example, every time you go into the kitchen you MUST do some pullups. Create an "ancor" or sorts, somethign that will always remind you to do pullups. If you cant do any more pullups with good form, do negatives. I thought negatives were stupid, but hey are not. They really help, and they DO work out your lats, making them stronger and allowing you to do more pullups with good form. Only thing to remember is, that there is no need to do pullups to faliure. You want to do a lot of pullups, but you dont have to do a lot in one go. I manage to do like a hundred pullups in one evening(many of which are negatives) but it takes me like 50 sets to do that, and I dont do these sets one after another. This routine really helped me, and even though I could only do one pullup with good form when I started, I can now do 4-5 in one go on a good day and 3 on any other day. This may not be all that great of a progress for an average joe, but I used to be extremely weak and sick, and for me it's an absolutely amazing progress.
2)Pavel recommends all kinds of "pumps" to help you do 1-2 more pullups or pushups when you are completely exhausted and absolutely cant do any more. I tried these "pumps" and amazingly it all works! For example, when doing a pullup, on the motion up, twist your hands from the inside, outside(like you are trying to screw your hands into the bar) and actually concentrate on that motion. I tried it when I was completely exhausted and I easily pulled myself up two more times!
Other pumps include tightening your stomach, gripping the floor as if you are trying to claw it with your nails(for a pushup), and there are some more that I forgot. Few people can do all these "pumps" at once but even doing one of them really amplifies your power.
3)Think about doing pullups when you are not doing them. Imagine yourself tensing your lats and doing the whole pullup motion when you are laying in bed before going to sleep. Picture it in your mind as vividly as possible. Do this for two weeks and your max pullup number should increase by at least one just from that alone!