Although my extremities are usually colder than the average human's (which I never miss to demonstrate @consultjohnwatson by putting my cold hands on his skin because he jumps so funnily then) and I do tend to have more troubles with my thermoregulation during cold weather due to my lean build, the heatwave now troubles me as well.
I have two main techniques:
A. Sheet, nothing else. And also restricting movement to a minimum, as excessive movement turns into thermal energy, heating you up even further. So I mostly lounge around the couch and let John do any movement like fetching me my phone, or Rosie when he is not here or too stubborn and tedious to do my bidding (which significantly increases during heatwaves). I already trained her well, she is very good at playing fetch.
B. Turning nocturnal. As the heat is highest during the day, I just stay awake the whole night to work, and sleep over the day. Which I also applied tonight, and as the temperature is now rising to 28 degrees I am going to sleep now. Works perfectly well for me, as I do not have a job with fixed working hours and I am my own boss. Another advantage is that you avoid most people that way, because humans tend to be diurnal.
Of course otherwise the typical techniques of increasing fluid intake, applying water to skin trying to use thermal evaporation as an advantage, creating aircurrents to cool down the room, et cetera. I also sometimes use endothermal reactions to cool down the surroundings, like mixing ammonium thiocyanate and barium hydroxide octahydrate, although the cooling is limited to the immediate surroundings of the beaker. Or using liquid nitrogen which is more effective at cooling a whole room, although you have to be careful with the fumes as it can cause suffocation with the accumulating nitrogen if the room is not well ventilated.
Or I just go and lie down on a table at Bart's morgue.