This convention that summer "begins" at the solstice is so weird and disconnected from the commonly understood definition of summer. Flowers bloomed months ago, it's baking hot, days are about to start getting shorter. It's already summer.

Keep the windows and curtains/blinds closed during the day, open the windows at night.

Sunglasses, cigarette and a leather jacket.

Soon?

Loose fitting clothes, big hat, bottle of water, shade.

Resting your feet in a bucket of cool water.

Works amazingly well. Change the water every so often.

Go outside and exercise as much as possible, while consuming sufficient water and electrolytes. Your body and mind adapt to the conditions you put them in.

Most people avoid the discomfort of adapting to changing temperatures as the seasons change. But if you just go for a walk outside on your lunch break and ride a bicycle around after work or something, you'll find you are much more tolerant of the ambient temperature.

It's finally hot enough that I broke out a sun dress.

Go to the Southern hemisphere.

We get a meal delivery service each week and they use the giant ice packs. So I've taken to just keeping two or three in my freezer and then sleeping with a pack at my feet each night or carrying it in the car while I do errands. And I get fresh ones delivered each week. A nice way to recycle the packs

Really depends on where you are.

Sun-heavy areas, long sleeves, long pants, hat.

In low-humidity environments, adding moisture to the air can help cool, which is why evaporative coolers work in places like southwest US/arid regions.

When out boating, my friends give me a hard time because I throw on a synthetic long sleeve shirt while on the boat with a real hat.

Put up a simple fan and sparkle water on your naked skin. That will bring your temperature down pretty quick for cheap.

Cons is that it's a little trying to be constantly spraying water on yourself and that it can be a little messy. It's also more convenient to do in a private space like your house. But it's way cheaper than air conditioning and you will be as cool. The principle of evaporative cooling is amazing.

Keep your living space cooler by blocking out sunlight from coming through the windows, preferably using shading on the outside.

Make liberal use of fans, they lower perceived temperature by several °C.

Wear loose-fitting linen clothing.

Use hats that provide shade, like a cap or something with a larger brim.

Go for a swim.

Take a warm shower before bed (counterintuitive but helps in my experience).

Try to create a breeze through your home by opening windows on opposite sides of the home, when the temperature is higher inside than outside.

Use sunscreen.

Always wear cool sunglasses.

Oh, you meant temperature wise? Silly me...

Clothes that leave plenty of breathing room, and avoid wearing clothes that are too dark.

As water is abundant where I live, I sometimes leave the tap running into a bucket in the sink and let any excess drain the normal way. That way I always have 10L of fresh mountain water as a heat sink. It's not as effective as ice, but it does work.

Move further north.

Living int he southern hemisphere

Close all your windows, and the blinds, watch Christmas movies with ice cream and cookies.

Always helps for me, lets me convince myself it’s winter and cozy

I don’t have air conditioning, other than window units that I try not to use to cool the place, but I do use them for dehumidification a lot when it’s hot out, and that helps an absolute ton.

Today (June 21st) is summer solstice

Solar powered fans

Take the plastic housing off of an upright column fan and turn it into an air blade.

I'm not joking, I have done this. It provides an amazing laminar (non turbulent) column of air that is so refreshing.

Install a hammock in your house ? Haven't tried it yet but seems like a good plan