Which means water. Hydrate, hydrate. Water, green tea, sports drinks... thankfully vending machines are just about everywhere in Japan, and the selection is always kind of exciting, so you should never be stuck without liquids. Convenience stores are on every corner (well, not in my rural area), but they also stock an impressive selection of drinks.
That little designer handkerchief will sure come handy. Not only to dry your hands or wipe your fingers after eating onigiri, but mostly to dab your face and neck while waiting for the train. Maybe use separate towels for those tasks...
No need to spend much time on hair and makeup, since it's a waste of time. I just let my hair air-dry, and for the first time in my life I do love my natural waves... or lion mane. As for makeup, if you must, waterproof it. Japan has an amazing selection of local brands, and weird things such as eyeliner that stays on for 24 hours. I must say, I love how Japan understands the need to stay pretty, even in the worst August heat.
Dress... not to impress
I used to be so self-conscious about showing my shoulders and getting disapproving stares from the elderly, but well, my Canadian body is not designed for this kind of weather. So yes, I do show my shoulders, I wear sundresses, and I do have a nice tan now. Being comfortable is key.
Become a vampire
Avoid being outdoors if you don't need to be, and avoid the sunlight. I go running late at night, and try to run most of my errands when the sun is down. Maybe it's just me. But this weather is exhausting.
And, lastly, dance to my favourite summer jam: