Rule number one: forget about training to failure. You can train close, within a rep or two, and occasionally (perhaps once or twice a month) push a set all out (read: A SET), but if you train to failure often, forget about achieving true pull-up power, and start worrying about your frayed and shattered nerves.
Rule number two: Vary your grip. Overhand pull-ups first and foremost but vary your grip. Doing so will ward off boredom, and train your neural pathways to a wider degree.
Grips worth using are:
1) Overhand or underhand, with or without thumbs.
2) Neutral grip--the best way to do these would be to drape a thick towel over the bar for maximum grip work.
Worthless techniques
(1) Wide grip pull-ups and 2) Pull-ups behind the neck. For some reason, bodybuilders think that a very wide grip makes for very wide lats! This is bogus for a few reasons, the first being the greatly reduced range of motion, the second being the greatly reduced leverage, and the third being the extreme stress on your rotator cuffs. Optimal leverage is extremely important in strength training.
Rule number three: You can vary the number of reps you use, just don't do it excessively. Pick a training goal. If it is maximum muscular endurance, stick primarily to high reps. if it is maximum pull-up power, stick with five reps or less (I prefer lower reps and use high rep Dbell or Kbell quick lifts for muscular endurance.