This is a work in progress!
vcgencmd measure_temp
This will only work with active cooling (i.e. a fan).
Edit the file /boot/config.txt
and add the following after the line which says #uncomment to overclock the arm. 700 MHz is the default.
arm_freq=1300
over_voltage=6
Save the file and reboot.
If your Pi doesn't boot due to some overzealous overclocking setting then press and hold the SHIFT key during the initial boot. This will override the settings on the config.txt