Firstly, you can keep your bedroom cooler by not opening the curtains or blinds during the day. Also, keep your windows closed during the daytime too. This is all, of course, much easier if you're not home during the day.
At night, do open your windows when the air is cooler outside and be prepared to close them again when the sun comes up. You can also hang a damp towel or sheet in front of your open windows and this cools the air as it comes in.
Prepare The Room
Unplug everything that could be adding heat to the room and you'll save on electricity costs too.
If you have a fan, it can be great to use one at night as it also adds white noise which helps when you're not used to the window being open. Some like to position a sheet from the bed over the fan so it blows right at you all night but I find this too chilly. Others advise putting something cold in front of the fan - a wet towel, sheet, flannel, etc - so the air is cooled further.
Prepare The Bed
Do use 100% cotton bed linen as it is a breathable fabric, so won't hold the heat as much, and it can soak up moisture - yes, your sweat.
Some advise sprinkling talcum powder on the sheets. I've heard cornflour mentioned too and, I think, both would soak up sweat and therefore leave you feeling less sticky. Even so I don't fancy the powdered route myself.
As we generally don't have large freezers in the UK, you could try putting your pillowcase - as opposed to all the bed sheets or your actual pillow - in the freezer and popping it on the bed when you're ready to lie down.
Make your hot water bottle earn its keep all your round by filling it with ice cubes and cold water in the summer and putting it in the bed shortly before you get in. If you don't have a hot water bottle (yeah, I know they're not 'hip' but they do an important job so I wouldn't be without one) you can freeze a drinks bottle and then wrap it in a towel and pop it in your bed.
Some suggest using a mist spray and spraying the bed and yourself but I've not found this very helpful. I've also been told to try putting a damp top sheet on the bed but I worry about the damp it might create in the bedroom as UK homes have a bit of an issue with this at the best of times.
Slow down your activity level before bedtime and avoid stimulating drinks, including alcohol.
While we guzzle gallons of tea in the winter, Brits prefer cold drinks on a hot day but it does seem that drinking hot tea on a hot day can help.
Before bedtime, have a quick, cool shower and wear either loose cotton pyjamas or nothing. This is a controversial topic as some say naked is the only way to be cool and others find wearing something cotton helps lift the sweat away from your body.
Do drink water before going to bed as sweating can result in dehydration. If you're the type who often needs a bathroom trip during the night just drink a small glass of water.
Keep a wet flannel, old socks, wristbands, rag, etc in the freezer and use this to cool down your pulse points at your wrists, on your inner elbows, neck, groin, ankles and behind your knees. You could take it to bed with you but do have another in the freezer ready as it will heat up quickly against your hot body.
A less damp option is a rice sock which can be used as a heat pad or a cold compress.
Hot air rises so it may be better to sleep downstairs on the hottest nights. If that's not an option consider sleeping on the floor to get lower.
Apparently, the heat and humidity can actually increase concentration so the way to take advantage of this is to start your day earlier - before the heat becomes a bigger problem - and to work hard.
I'm trying all of these tips at the moment but would love to know if you have any more ideas. Please do add them in the Comments below.