What I have found to be the most useful for my students with this trouble has been for them to inhale through the reg (obviously *S*) and then exhale out your nose. This is not the correct way to do the skill, but it will give you a chance to get comfortable breathing without your mask on underwater. After a few minutes of this, and being comfortable with it, you should be able to relax and begin to exhale out your reg without inhaling again through your nose.