If you have cool nights, setup fans up at night to bring the house down to a lower temperature. Close everything up in the morning when the outside temp starts rising above your inside temp. If your place is insulated reasonably and there's no excessive sun from windows, it will stay cool for the day.

Protip: Setup the fans in all rooms on one side of a chokepoint in your house/apartment (stairwell/hallway) to exhaust, to encourage airflow. Open up all the windows on the other side for intake. It'll also help reduce pockets of hot air left over from the day before.

Wet T-shirt and sitting in front of a fan.

We naturally cool our bodies by sweating.

You can sort of hack that process by getting a t-shirt wet, putting it on, and allowing the moving air to help speed up the evaporation process.

WARNING: NOT FOR USE IN HIGH HUMIDTY.

Adding more water to an already hot and humid situation risks a Wet Bulb.

If you have a freezer and a fan, freeze a bunch of water bottles and then put them right behind your fan blades for a cheap AC-like chilly breeze. If you have enough bottles, you can cycle through them and refreeze as they thaw out.

Run cold water over your wrists when you're starting to get hot.

When I worked in a restaurant kitchen, we used to soak rags with water and freeze them in the walk in freezer, then once it's nice and frozen we'd wear the rag around our necks.

There's large blood vessels in the neck feeding your brain, so if you're able to cool down the blood there, it'll spread to the whole body surprisingly fast.

I actually managed to get cold in hot humid july summer in the kitchen with that method.

Block out the heat and sun during the day. Have everything open during the night, with a tactical fan placed wherever it helps the most.

But this only really helps the first few days. After that it's down to accepting the situation and being so tired that you fall asleep anyway.

I used to not have AC. I found the best strategy was to open up all the windows at night and let the cool night air in, and then as soon as I wake up I close all the windows, curtains and blinds to trap the cool air in and prevent the sun from heating it up through the windows. If you live in a house that has a basement and central heating, you could add some intake ducts down there and turn your furnace on to fan only mode to circulate the cool basement air into the rest of the house.

Windows open at night. Curtains or blinds shut during the day.

At night cool your house down by opening windows and using windows fans. I have a two story house. Heat rises so I'll place window fans blowing out in the upstairs windows and fans blowing in upstairs. In the morning close up the windows and close curtains and shades over the windows that receive direct sunlight. If you have a room that gets hotter than the others shut the door to that room or hang a curtain over the doorway. My hot water tank is in a first floor room so I isolate that room and leave the windows open. Have a ceiling fan? Make sure it's blowing in the right direction. Most have a switch so you can alternate direction it blows the air. Not always practical but soaking your feet in cool water will lower your body temperature. Much like coolant cools a combustible engine car your blood circulates through your body distributing heat. Personally I avoid using AC while driving as much as I can. It's my opinion that when we get used to such comforts we suffer without them. I do have a window unit but use it only on the hottest nights. One last thought. Some lights or appliances in your house may give off a lot of heat, feel around them to find which ones do and switch them off. You can't do anything about your refrigerator (gives off a lot) I had a plasma TV that felt like a space heater and also used a lot of electricity

Around here, Portugal, were every Summer the temperature exceeds 40 C for at least some days in August, we have outside rollup shades on every window, so one of the tricks is to keep the shades down and and the windows closed during the hottest and sunniest parts of the day, at the very least the afternoon.

Then at night you open the windows and let the cooler night air in (even better if you do it early morning, around sunrise, which is the coolest time of the day).

Note that this doesn't work well with curtains or internal shades, because with those any conversion of light into heat when the light heats the shades/curtains (as they're not mirrors and don't reflect all light back) happens inside the house and thus that heat gets trapped indoors.

Go to the movie theater or supermarket for some AC.

Spray water (fine mist from a bottle) on the inside of your windows and use it to stick aluminium foil to the glass, shiny side out. I do the top ⅔ of the north-facing windows (I’m in the southern hemisphere) each summer and it reduces the indoor heat significantly.

Those soakable cloth neck-wraps work as a cheap personal cooler($5).

The Coolify2 works as an expensive personal cooler(~$200).

If you have a fridge, freeze 2 litre ice-cream containers filled with water to make large ice blocks. Then put the block in a tub to melt, and sit your feet on it to stay cool(budget-mode, $cost of tap water)

I have a basement where I swear it's 10°C colder than upstairs.

Your feet and head are both very vascular, so cooling them will help lots to cool the rest of you.

Head -

Ever been buzzed or bald before? If no, now could be the time to give it a shot. Worse case scenario, you look like shit and let it grow back to whatever's the shortest length that looks decent. Bonus: you'll save a ton of time and money on hair cuts/care.

Keep a container of water water and washcloths in your fridge. Take a cloth out when it's time to veg on the couch, and slap it on your noggin. When it dries, grab a new one. *recommend not throwing used ones back in the water w/o washing first, or your water will get nasty fast.

• If you decide to go buzzed and have never done it before, PROTECT YOUR NOGGIN/SCALP FROM THE SUN. Burns up there hurt like a mofo.

Feet -

This is trading heat discomfort for wet sock discomfort; but if that's a fair trade, then... yeah, wet your socks with cold water. A tub a cold water at the base of your couch can give you something to dip in while you're watching TV or something. Same spiel as the wash cloths - keep your socks/water/tub clean and don't reuse without washing first, or you'll get yourself trenchfoot or some nastiness.

Also, if you're in an apartment that disallows window units... they fit great in a fireplace, and the hot air just vents up the chimney. Your lease likely doesn't say anything about fireplace units. Just sayin'. Just make sure to seal the edges really well so hot air doesn't leak back into your living space.

By living in a country with no seasons

If you're in a humid environment and you have a way, dry the air. It makes any other cooling thing you do more effective.

I grew up without AC in Florida.

Sit in the shade with a fan, be still. If in the shade and a breeze, and not moving around, a pretty high temperature is comfortable.

Go to places with cold AC or to cold springs to get cold, it will last awhile after you leave.

If it gets cool at night, open your window at night to let in the cold air, close it in the morning to keep that air in for the morning. But once it heats up inside, you are better off with ventilation - open windows on both sides of the house and run fans, to move air throughout the house.

If it's dry where you are (it doesn't work here) get wet and let evaporation cool you. Even here you can get wet and stand naked in front of a fan you will get cold.

STAY HYDRATED.

I didn't go through all the comments so sorry if thus is a duplicate.

Last year I learned that having a wet (not soking-weat) towel on your feet or your belly can keep you cool during the night. If you struggle falling asleep without a blanket and your're to hot to sleep it might help.

Keep windows open at night and into the morning

Fan pointed straight at you, on max blast through the night

It's significantly worse if your house is humid, and dehumidifiers increase the heat, but they're still worth it

Buy a dual hose portable AC, that's what I did when I lived in an apartment that would get VERY hot no matter what. (Actually, to save some $$ I got a single hose and modified it to a dual hose, but depending on the specific model and the tolerances they built it to, it risks short cycling and possibly burn out)

They're still not nearly efficient as a window AC, but far far better than those single hose ones

If you can fit a window AC do that instead, if you are able to make modifications, a small mini split/heat pump system would do wonderfully. Though I have heard that they make mini splits that go through small windows rather then needing to drill through the wall, so that might be an option too.

The other tips and tricks are nice, if you have exhausted all other AC options and simply can't have an AC at all (Which is mostly due to cost, dual hose portable ACs are pricey) but they really don't compare to an actual AC system.

Get a dehumidifier.

Ensure ceiling fan rotation is switched on summer mode.

Bag of ice in the freezer to chew on and always have in a drink. Ice trays are no good.

Waterhole.

Office job.

Keeping lights off/low can be psychologically "cooler" for some.

Bed with wet hair or damp towel will help sleep faster.

Wash regularly to scrub "ick" layer and keep fresh skin on top.

Pub.

Ice pack vests. Can DIY. Fucking amazing.

Sit around in your undies and pretend you love it and eventually you do.

Open the windows at night and close them in the morning. If you have many electric devices, like I do, turn them off or put them into a dedicated room. With many, I mean lots of devices.

If you have a basement, stay in it. When it was 108°F here in Seattle the other year, it was in the mid to high 70s in my basement office. If you do not have a basement, am evaporative cooking method can cool you a bit of you are in a dry climate. You could also hit up a pool or grocery store or movie theater. All those will help you cool off.

Check Craigslist for a portable AC unit. I was able to snag a free dual hose portable AC unit a month ago. The hotter it is though, the higher the prices are on Craigslist.

If no ac then stay in the basement while I reasonably can. Drive with windows open. If my work has no ac then the business closes. The material we work with has to be temperature controlled.

Buy a portable AC unit and install it in your bedroom or living room window.

EDIT: I have this one that works well at least on a single room: https://www.amazon.com/Vremi-000-BTU-Portable-Conditioner/dp/B084H4B6NB?th=1