@Tatsu: Hm, for that costume I would go ahead and keep the revealed hip, but most importantly add SHOULDER PADS to that outfit. A lot of girls seem to be iffy about the whole shoulder pads thing, but trust me they definitely add to the masculinity. Guys shoulders are ALWAYS significantly wider than their hips. (And fortunately that character has a sort of effeminant look to him, so showing off the hips and keeping some femmininity probably won't be that bad.)
@Sweet Oneesama: Yeah yeah, I empathize with you there. Oh man, KH2 Riku is going to be a PAIN to pull off a convincing crossplay. Dx His shoulders are bare so you can't use shoulder pads, and his midriff is exposed (a little) so you can't pad your waist... hrmm...
My only inexperienced advice would be to wear those tight spandex slimming shorts underneath, and wear really really baggy pants, set low on your waist. Then do pushups like no tomorow. xD Good luck, good luck! (Big skater-shoes will also help with the illusion, I think.)