CKE18 - Bikini Balance

Some Japanese girls we shoot have terrible balance but Shiori was a natural at sitting on the ball. I was curious so of course I asked why! Shiori has her own balance ball at home but admits she doesn't use it as much as she should. Her plan was to tone up her stomach before summer so she could look good in beach photos with her cute friends. She said she needed to keep working on it but I thought she looked great already. Have any advice for her?