I’m having huge success undervolting the MacBook Air with CoolBook.
Quick summary of what this means: Undervolting makes the CPU run at the same speed as usual, but with less power. This makes it generate far less heat and use far less electricity. Some CPUs can tolerate it and some can’t — each manufactured chip is slightly different. By default, they’re configured to use a pretty conservatively high voltage, but if you find the limits of your particular CPU, you can get big gains in battery life and low temperatures.
So far I have 800 MHz, 1200 MHz, and 1400 MHz all running at the minimum possible voltage of 0.9000 V, and 1600 MHz running at 0.9625 V (it kernel-panicked at 0.9500). Everything’s stable so far with CPUTest at these settings.