What is good technique when operating a fire extinguisher?

Aim the fire extinguisher at the base of the flames, and sweep back and forth

More specifically:
Trigger the fire alarm if not already done.
Have others leave the area.
Make sure you can get out safely and not be trapped.
Select a fire extinguisher type according to what is burning.
Pull the pin
Aim the nozzle at the fuel
Squeeze the handle
Sweep from the edges toward the center
Repeat, as necessary, with another extinguisher, if you have one.