During this current heat wave I've been struggling with heat related discomfort in my sits. (A/C isn't an option at my home) Any advise on dealing specifically with this, I'm sure the people who mediate in the east have it worse.
We must learn to thank the physically uncomfortable things in life, because it is a great teacher.
The retreat center in my city is right beneath the primary flight path to a busy airport. New meditators on their first retreat often get very irritated by this - because not only is it hot and sweaty, especially if they aren't from India, but they didn't sign up to deal with this infernal din from above.
We may not realize it, but we often live in our thoughts and memories and not in the present moment. Our memory of comfort is what comes in conflict with the present discomfort, not the physical pain itself, since it is the mind that gives meaning to pain.
When irritation swells up in us we must immediately thank it, and investigate it - who is getting irritated? Where is the clinging coming from? What exactly is uncomfortable? What is getting hurt? Is it our senses or our expectation? When exactly did the irritation begin? What happens to our attention at the moment of irritation? When does it end? How soon does it begin, and how soon does it end? What is its nature? Is it constant or does it arise and pass in waves? Can one see the three characteristics of anicca, anatta and dukkha in the irritation? and so on.
When the first aircraft flew above me, I was mildly irritated by it, but seeing that this was a great teacher who had come to guide me, I began to practice loving kindness towards the passengers in every aircraft that flew above me after that. Soon it became a habit, and so while there were some who would audibly sigh and hiss at the aircraft, I would be enveloped in a warm glow of love several times every hour.
It isn't the physical reality but our reaction to it that matters.
Try meditating outside sometimes if you can, making sure you are safe and secure. That's how the forest monks did it back in the day of Buddha-lectures-at-full-moon.
The most important "technique" is just being aware (vipassana). Be aware of everything recursively as it occurs including any scattering of awareness itself. Eventually everything will calm down and you will be quite alright and cool.
Also, take a cool shower and eat less (Buddhists do not eat past noon meal, thus reducing our sexual desires, creating a detox state, and many many positive effects including heat reduction).
You could try doing walking meditation prior to sitting, at a fast pace. This would accomplish two things:
1) The walking itself would make you hot, therefore when you sit down, you would feel relatively cooler.
2) Doing walking meditation at a fast pace arouses energy, thereby making the drowsiness that comes along with being hot less of an issue.
Also, if you have a basement you could try sitting down there, instead. Basements are generally cooler.
Dealing with Heat: use a large and quiet fan on a slow setting. My Meditation Teacher said, "there must be movement of air." Ceiling fans are amazingly helpful, if the ceiling is high enough for one in your room.
Wear as little as possible. In your home you can wear a loose skirt even if you are male. (Perhaps in other situations people would not mind either.)
If the humidity is low, you should be comfortable in this way even at a fairly high temperature (90F / 32C). Most likely the problem is humidity, which impedes your body from cooling itself. The fan will help. If it is humid, dehumidify the room, or keep it closed to keep humidity out.
Some fresh air should be brought in while you are sitting, if at all possible. Cold is not so hard to deal with, as blankets wrapped around are comfortable. I used to live at a Retreat Centre, where outdoor temps varied from well below freezing for months, to "blood heat" (body temp) with humidity at times for weeks in the summer. For years we had no A/C, then only a large window unit for the whole house. But something can always be done, and it does not last forever in any case.