This is something I used to practice on a regular basis, when I was in US Army deep sea dive training. Always practice new techniques ABOVE water till you understand all the variables innvolved. First, if you haven’t already, try filling your lungs with as much air as possible. While holding your breath suck more air into your mouth and force it into your lungs as well. In time, you will expand your lung capacity but, you have to pace yourself or you’ll injure yourself. Once your lungs are filled to capacity, in your mind, sing a song to yourself. Make it a song that you know word for word, (I chose Beastie Boys’, Paul Revere) and try to make it to the end of the song before letting your breath out. Now, speed it up annd try to sing it twice. Remember to remain totally relaxed (I often layed in bed without moving) so you don’t waste energy or concentration. When you attempt this in water; you will have more air intake capabilities and your body will start to operate better as your oxygen saturation begins to drop. Now, sing that song while swimming uderwater. How far do you get? Document and repeat; this is more than practice, its training.
This subject has always intrigued me but I find it risky and dangerous. David Blaine practiced and prepaired for his stunt/s for months with the help of Navy Seals and his physician, not always with a pleasant outcome. But I admire his determination.
Research fully and advise your physician.
Just a little precaution here. Breath holding under water becomes dangerous after more than 30 seconds to a minute depending on the person. I used to do breath holding exercises and experienced a number of the warning signs like change in vision, ringing in the ears, etc. I could at one time swim 100 yards in a 25 yard pool (turning and pushing off underwater). I nearly blacked out several times. A friend actually did black out once and if someone hadn’t been there, they would most likely have drowned. I gave it up after that realizing it is too risky compared to any possible benefit. You should look into the physiological effects of extensive breath holding if you still think it is ok to do it so excessively.
What you are doing is a lot like standing on a fit ball. Almost every person who has set a record for time standing on a fit ball has blown out a knee or two. And what does standing on a fit ball prove or improve? I stood on one once and could tell immediately that it was stupid. Want more to think about? Check into how many free divers drown every year.
What exactly do think training to hold your breath will do for you? You do know that when all available oxygen dips below a certain concentration, the energy system in your muscles shuts down. And the brain will make you black out in an attempt to get the natural breathing feedback mechanisms to start back up. This is not something that cannot be improved. And if you find out just how long it takes for your brain to react this way, I hope you aren’t underwater at the time.