I really liked the period when I did freediving, and practiced to hold my breath. Doing "CO2-tables", almost like intervals except holding your breath. (And as in running, you don't run a marathon every workout, so didn't "push" it to the extreme these breathing sessions). A CO2-table for instance could have you hold your breath for 3 minutes, breathe normally for 90 seconds, then again hold for 3 minutes, then only 80 seconds break etc, decreasing the break each time.<p>It's quite a sensation overcoming the "need" of breathing. To truly learn that the feeling of having to breathe is only because of CO2 buildup in your body, not because of lack of O2. Mind over body.