If you live straight under the roof and it's hotter inside than out: close the blinds, open the windows.

Otherwise, keep everything shut, close the curtains, get some stick on sun shades for your windows. Open everything at night, then keep an eye on the heat in the morning and shut everything when it gets hotter outside than inside.

Have lukewarm (NOT COLD) showers. If you have a bath, fill it so you can cool down every now and then. Also fill a washing up bowl to put your feet in.

Get a fan. If it's not too humid, hang a wet towel in front of it or pile some of those bricks you use to cool a cooler in front of it.

Wear linen, viscose or modal. Don't wear trainers.