Um... that costume could prove problematic. Why? because the entire back is open, so not even a strapless bra would do, and corsets are right-out. ^^; The best I think you could manage would be to sew some sort of falsies into the costume itself, since there's no reliable place to strap it tightly.
Also, I sure hope you're skinny, because that costume leaves little to the imagination around the waist and the entire back... if you do it at all, you'll also probably want to wear the shorts ABOVE the hips, so you can pad your hips, because if you're a guy chances are you don't have hipbones anywhere near that curvy.
And yeah, birdseed is pretty good for holding the right shape, but be warned: it smells funny. Freezing the seed before you use it might help (it sanitizes the seed somewhat), but nylons will not hold in the smell at all. I wish I could recommend attempting to wash them, BUT, trying to hold seeds that small in a strainer would be hell, and they'd be nearly impossible to dry. >_<
Rice is your next option. Also, it may sound corny, but double balloons (one nested inside the other) filled with water are effective. Just make sure they are held securely in place... I shouldn't need to tell you why. >.>;