11 marine style pull-ups will get you a good score on PRMC mate so as far as improving goes i'd concentrate on something else, but your the boss. Bent over rows. This dumbbell routine will help develop your biceps and your upper back muscles required for performing pull-ups. Repeat for three sets of 10 to 15 repetitions.
I would agree with bradfordbulls here, 10 or more is bang on top marks and you should probably focus somewhere else. But for getting more out of your pull ups try adding weight so that you have build your way up to 6 pull ups and above again.