When your in an intense game, and you want to send 3 workers to gas, you usually don't have the time to find only workers that don't have minerals. So sometimes you will send some workers who might have mined a mineral and those minerals will go to waste.
What I figured out was that, if you order a worker to go to the geyser and then press "C" (return cargo), it will return the minerals and then proceed to the previous action you gave it. So that means if you take 3 workers and send them to gas, and then press "C", any workers with minerals will return them first and then go to collect gas.
In case people haven't realized this as well (forgot to update this when I realized you could use this trick for transferring workers as well):
This is also very useful for maynard workers.
Right click on mineral patch at expansion
The workers will return the minerals to the nearest Nexus/CC/Hatch and proceed to the expansion