-Put your water in glass jars. They stay colder. -Put your glasses in the freezer. There's nothing like a frosty glass of whatever. I like to add a little water and empty it before putting it in the freezer to give it that extra icyness.
They're obvious, but it's weird how long I went without doing it.
My tip: walk on the shady side of the street. There are some people who refuse to believe that it's actually cooler in the shade than in the sun ("it just doesn't make sense!"), but it really is.
Also: when you must work in the heat, don't go into air conditioning during your lunch break. You won't want to come back out. And contrary to the ice-cold advice above, I'd recommend keeping beverages during the heat of the day cool rather than frosty, so as not to shock your body too much!
Also: don't slather on the anti-perspirant too thick. Sweat may be gross, but it's also how we keep cool.