After breathing, there is perhaps no skill more fundamental than mask clearing. While other skills, such as gear assembly, are a requisite part of getting in the water, they technically could be done by someone else (although this is not recommended). Mask clearing, however, is a solo skill, and the inabiility to perform it will easily ruin a dive.
Once mastered, this is a skill you will use immensely. I frequently perform mask clearing not just when water leaks in, but also when I insufficiently apply defog. Mask fog can ruin an otherwise good dive, but with, ahem, fluid mask clearing skills, one can quickly and easily remove fog as it appears and continue with the dive.
Mask clearing can be broken down into 3 simple steps:
To begin, place your left hand (fingers or heel) on the top part of your mask frame to hold it securely against your forehead. Some divers use two hands, but this is unnecessary, so I would recommend practicing with one hand to minimize movement.
Prepare to look slightly upwards by tilting your head.
Begin exhaling from your nose until the water has been removed. As you begin exhaling, continue looking up to remove all traces of water.
Let's go over each step:
Holding the top frame of your mask ensures that the air exits through the bottom of the mask, where the water is. Otherwise you will just be blowing out air, and end up needing multiple breaths to clear the mask completely.
I say "prepare to look up" rather than simply "look up" because looking up before exhaling, at least for me, lets water inside your nose. This is not a pleasant feeling and can increase stress levels. Only start looking up once you are exhaling through your nose.
Why do we tilt our heads up? It is similar to tilting a beverage glass when drinking from a straw: tilting allows all remaining liquid to move to the bottom-most corner, ensuring that it all can be removed. See the accompanying image.
Note for instructors: For demonstration purposes, you need to indicate to the student that you will tilt your head up. This is most often done by using the right hand to indicate where you are looking.
Exhaling actually removes the water! Remember to look up to get the last bit into the bottom corner! Try to get it all in one exhale.
While the steps are simple, this can be a challenging skill, mostly due to the unpleasant sensation of having water on your nose and face before clearing. Like most things, repetition will increase comfort levels and reduce stress, leading to more relaxed diving.