I have tried mulitple ways to improve my pullups including multiple sets of low reps, weighted pull ups, and sets of 10 spread out through the day. It was hard to see progress, so I would discontinue prematurely, thus no results. A budy of mine told me about another way.

If you can already do at least one pullup start a Piramid going down from 1 (pullup) to 1(10 push ups) with 10 push ups inbetween pullups no rest and the last pull up finnish with a negative 30s or greater. A day of rest and start the first set at 2 pullup and the second set will be 1 pullup at next time. Progress till you can not complete the entire piramid and the next session start the next session with the first set intended to be the number you could not start but at 1 pullup. Each time you perform the routine, add 1-3 pullups to the first set till you reach your set you orignally could not complete. The glory of this routine is that you see progress every time you perform the routine and you make goals that you can meet. You will have to do this routine every other day. You will need that full day of rest for recovery. When you start your routine at 10 pullups you will be able to do 20 pullups. At my routine starting at 11 I could do 25 pullups. It works! It does take time and hitting the pullup bar three times to four times a week for possibly months to reach your goal. If you can already do 10-12 pullups, it should only take two months to get to the point that you can do 20. This also is much more effective for building the upper body beach musles that Bench and curls alone (a waist of time in my opinion).

This is the routine with displayed in non paragraph form:

Start:
1 pullup then 10 push ups
Rest Day
2 pull ups, 10 push ups, 1 pull up, 10 push ups
Rest Day
3 pull ups, 10 push ups, 2 pull ups, 10 push ups, 1 pull ups, 10 push ups
Rest Day..
Continue increasing adding another the first set with a value 1 greater then the last untill you can not complete the entire routine. You will reach this point in week 1 or week 2.
Rest Day
If the last routine you completed started at 7 then you will start the first set at 1 and continue at 7 pullups
Rest Day
Add 1-3 pull ups to the first set, then continue at 7 with one day rest inbetween till that first set is 8 pullups.
Next session start the first set back at 1 and work up to 9 and so on (this is the process that takes months).

Another great aspect to this routine is that you build muscle that stays. You will not lose 20lb of muscles in a month like if you stopped doing heavy squats (hint hint... if you want to get bigger do Squats!). Keep the routine simple, allow yourself to see progress, persistence, and will produce results! Do not skip rest days to reach your goal faster, because you will hurt your progress and you will quit prematurely, never reaching your goal. One session, or one week of effort will do nothing but make you sore. Months spent reaching small goals every session with a day of rest in-between sessions, will produce results!