Zach, the real key to get a high olie is to practice it over and over again.
Also, copy everything Keith Lyman does.
I have also seen a real difference in my olies since I became obsessed with P90x polymetrics. See the P90x forum in non-wake.
I also practice jumping as high off my indo board as possible and trying to land on it (for balance). Doing this on the ground works well too.
Some people double pump their olies; where you do a small pump before you do the olie, but it is too inconsistant for me. Especially when you are trying to olie a solid steel drum like the picture above.
Simply put, I start with my legs slightly bent, crouch 3/4 of the way down, pop off the tail of my board (really hard), suck my knees up and then land.
I would also say that using the slingshot is like cheating. That pick was taken on my second time riding that board. The pop is amazing.