First and most importantly, muscular failure has nothing to do with strength development. So, ending your sets a rep or two short of failure will be an important step forwards.
Secondly, since you're already rather advanced in bodyweight exercises (40 pistols?? Are we talking about the same movement??), I'd really like you to find some kind of external resistance. Push a car, lift a couch, toss some rocks or books into a backpack. A very important note here: You will get stronger by lifting some kind of weights.
I'd like to see some kind of pull-up. Trees and rafters come in handy for this. Or monkey bars, if you have a schoolyard nearby. Also, some kind of rotational ab movement, like rainbows or russian twists, would give a more well-rounded ab workout (along with some kind of lower back work).
Another thought would be to take an EDT outlook on things (see Charles Staley's basic EDT workout: http://www.T-Nation.com/readTopic.do?id=459765 ) Essentially, try doing the HSPU's for a total of 10 or 15-minutes, and note the total number of reps. Next workout, all you need to do is achieve at least 1 more rep in the same amount of time. Something to consider.