The recent hot VS cold polls have made me realise that a lot of people have no idea how to cool down.
As someone from a hot country that's regularly on fire, here's some tips:
WATER IS YOUR FRIEND! WATER! IS! YOUR! FRIEND! You can transfer SO much heat into this bad boy! You cannot cool down without water!
Wrists under the cold tap. Splash your face and the back of your neck. Fan yourself.
In some countries you can buy a little handeld fan with a water sprayer.
Damp tea towel around the neck. Stick an ice pack in there on hotter days.
Half fill a water bottle with water, stick in freezer. If you use a bottle with a straw, make sure it's lying on its side with the straw side up and out of the water. When frozen top up the rest of the way with tap water and off you go.
Desperate to cool off? Wet T-shirt. Sit in front of a fan. This will nuke it, just don't get hypothermia and don't fall asleep like this.
Cold showers are also your friend in summer. Some people get psyched up by these. Personally, I sleep like a baby, so I'm good to have them before bed. Just keep in mind that it takes a bit of time for the cool to circulate, so your body will tell you that you're colder than you actually are. I find that when I have cold showers I need to step out of the spray when I think I'm cold... I'll just wait, and thirty seconds later the temperature has evened out and I actually need to step under again. Rinse and repeat until you maintain coolness even after stepping out for a bit.
If you can't do cold showers, turn the cold shower on anyway and just stick your arms under. When they're cold, lift your arms up above your head. The sensation of cool blood draining into your body is fucking weird and kinda unpleasant but less unpleasant than being hot.
Feet in a tub of water with ice. Blood naturally flows to your extremities when hot, so take advantage of this. If you don't have a tub of ice water, sticking a wet rag on your feet in front of the fan works too, it's the less powerful version of the wet T-shirt.
Drinks lots of water but make sure that water has electrolytes as well. Stay in the shade.
Keep air circulating. Fans don't actually cool rooms down, they just help transfer heat from your body to the moisture on your skin or the air via evaporative cooling.
Block north facing windows early in the morning so the sun doesn't get in. If you're in the northern hemisphere, this is opposite for you. Keep in mind that if your home is brick, the bricks will still heat up and slowly release heat into your home even after the sun goes down so this will only do so much.
If it's hotter inside than outside, close all your windows but two, making sure they're on opposite sides of the house/unit you're in. Point a fan out of one window, making sure that the doors between the rooms with the open windows are all open. This will help create a mini pressure system in your home, pulling cooler air in and pushing the hotter air out via the fan. Bonus points if you can get that fan high up where the hot air rises; even within a single room the top is much hotter than the air by the floor. Adjust the amount of open windows based on how many fans you have, but generally you want more windows with fans open than windows without fans to keep the pressure correct.
Obviously, use your common sense for these. Not everything WILL work for you, just use the stuff that does and adjust what needs to be adjusted. Some of these will be impossible to use in the workplace but others you can still use. Others are best used at home. If humidity impacts your ability to use any of these, get a dehumidifier if that's an option, or use more ice instead of evaporation.
Also keep in mind that the skinnier you are, the faster these will work. More fat means more insulation, means more heat, so you may need to be more patient with some of these or use them in combination.