get lighterweight fabrics.
i have two shirts i love that i got at a preppy store last spring that are ruffly and very voctorian, but a very light fabric,perfect for summer.
also, put in some horsehair braid in the hem of your lowest petticoat. skip the bloomers. this will allow for air circulation under your legs.
go for lighter materialed dresses, and short sleeves. you can get away with a style i have seen in victorian maiden a lot, the really short slightly off-shoulder puffly sleeves. try that with a square neckline and two ribbons crossing behind your head. either that or you can try your hand at making a tasteful jumper that does not have a shirt under it.
go for either light stockings or thin knee-high socks.
or just be glad. the women back in victorian times had much more to wear in summer. :P
but i have worn things like this here, and it gets hotter than that, and recently very humid in summers. :P i wish you luck.