I have a special technique that I use to keep cool. I have a special cream, it's like a menthol cream, "Pitari Sweat." You put it on the back of your neck and a few minutes later, you feel great!
I go to the sento and eat ice-cream from the convenience store. I eat the "Giant Cone" at least twice a day.
I always have a fan and mini-towel, and I take a shower twice a day. My fan is sensu-style, with a scent that refreshes, better than uchiwa fans because you can make it smaller.
I sing worship songs to keep me cool, Christian songs. Actually, it keeps me cool because it's in the park, under the trees.
Wear shorts and sandals to work and change at work. I also keep the air conditioner at a steady 18 degrees all day. Not very environmentally friendly, I know.
I try to find a river somewhere outside of Tokyo and sit under a tree. Failing that, I go and find a nice bar underground and have a cold beer.