re: Good routine to improve pullups?

I'm about the same age and have been stuck in the 12-15 rep range for the past decade or so.

About a year ago, I was visiting family and decided to try one of those outdoor workout areas that you see at parks. The only upper body stuff I did that day were pushups, pullups, and dips. For the pullups, I did 100 in increments of however many I could do at a time. I remember on the last set barely doing 10 (with form getting worse on every rep). That led to a strain in my left elbow that lasted for a couple of months. Gradually build up your reps and don't do what I did (doing way more than usual).