Disabling the HDMI port
Make sure you can get SSH access once this is done first so you have the ability to turn it back on again should you need to!!!
sudo nano /etc/rc.local
# Disable the HDMI port (to save power) /usr/bin/tvservice -o
This will disable the port once the RPi has booted (its still active during the power on startup process).
If you want to temporarily re-enable it you can use this (in our test it re-enabled the screen but it was then just blank so not much help, but maybe there's another command to kick it back into life?):
Disable the LED
sudo nano /boot/config.txt
# Disable the LED (to save power) dtparam=act_led_trigger=none dtparam=act_led_activelow=on
Minimise software tasks
Unless you need something don't install it as background tasks cause the processor to consume more power servicing them.
Under Clock CPU
Apparently doesn't make a significant difference to idle power, but might be worth experimenting with if power consumption is critical.