When I started hitting sliders I had the same problem (I think we all do). Anyways, I found getting the board at 90 degrees to the slider makes it much more stable and easier to control. And then you move on from there.
As far as Ollies, practice makes perfect. Do you skateboard? I try and ollie like I would on a skateboard, off the tail. Though its not really the same thing, I just found improving my ollies on a skateboard, made them better on a wakeboard.