Colombia Sportswear has come up with a new type of clothing that can actively cool itself, and you, when it gets wet. It's got some kind of fancy chemical that chills down when exposed to moisture, to the point where if you use enough of it, you can potentially freeze yourself.
Okay, so Columbia's Omni-Freeze Ice clothing won't actually give you frostbite, but it does provide a noticeable cooling effect. The fibers of the clothing are bonded with a special chemical compound that activates when wet to lower its own temperature by a noticeable five degrees or so. It works with your own sweat, or if you're desperate, you can just dump water over yourself for increased effect. The fabric itself is also designed with flat fibers that increase its surface area by over a third, which makes it that much better at whisking away body heat.
Since the cooling effect depends on a chemical, it's unfortunately not going to last forever. After somewhere between 50 and 70 washes, all the cooling stuff will be gone, but that means it'll last you for at least four years or so, if you do your laundry as often as I do.
You can expect to see clothing with Omni-Freeze Ice technology available sometime next year, and until then, I think the safest course of action is to just not get any exercise at all. At least, that's my plan.