Here's what works for me.
I put my hands slightly wider than shoulder width. I also splay my fingers and try to put the majority of weight on my hands.
I also make sure that my back, especially the lower back, is not sagging in any way. I try to keep my hips in an anterior tilt the whole time. This really stablizes the lower body and works the core something fierce.
Finally, when coming out of a roll out or plank, it's tempting to let the weight of your body bring you back to parallel. I resist this temptation and really try to guide the ball back out.
All this being said, when Cathe asks us to raise one leg off the ball, I'm more than likely going to start tipping and rolling all over the place.
As they say, practice makes perfect, so keep trying!