Have to agree with everyone below who said it: get forms and never look back. There is nothing more feminine than the jiggle as you walk, or the way they will deform if you bump into something or hold something to your breast. When you lean forward they lean forward too, and they give a much more natural shape than rice or lentils (I used to use cous cous, flour kept leaking and rice/lentils were too coarse). Water balloons are not really great, they want to be round and so stick out in a strange way.
Nope, definitely need forms!