Buy a pair of riding tights in one of the "cooling" materials.. you can often find good deals on ebay
if you stalk the auctions long enough. I have several pairs of them and I will never go back to cotton tights for summer riding ever again. The difference is amazing.
I ride in t-shirts that I have cut the sleeves and neck out of to improve air flow.. if the bushes wouldn't scratch the crap out of me, I would probably ride in just my sports bra in the summer.
The sponge I carry attached to the saddle is my favorite thing for hot weather riding. Not only does it allow me to cool Dream, but I have also been known to sponge water on myself as well.
I second whoever said a nice, vented riding helmet is a must and I personally tuck all my hair up into it.
Unlike most, I actually ride in the heat of the day on purpose. I need both Dream and myself to be used to the heat, as the weather doesn't give us a break during competitions, so we have to be ready for it.
Sure hoping our summer is as kind to us in terms of temp extremes as our winter was!!