Rotate her body forward. Either her entire body, or just her hips (which will rotate her entire body). I think the problem is that you are posing her perfectly upright. But she's not riding a tricycle. She's riding a motorcycle. She shouldn't be perfectly upright, she should be leaning over. And the way to do that naturally is to rotate her entire body forward. That will let you put her butt in a more natural position, as well her arms, stomach, etc.