I've had a go and found that reducing the glutes size to minus 1 is a big help in extreme poses.

The top two renders show the improvement from the rear without detrimental effects to the side view. My pose is also less extreme which also had an effect.

Neither competes with phanton3d's examples.

The render on the bottom, pardon the pun, shows that reducing the glutes as on the left hand figure, does ruin her arse in a less extreme position!