Great to see another freediving enthusiast!
When I started freediving in 1989, information on freediving was scarce. In fact, I recall that hyperventilation was the theme in order to extend breath hold times. Fortunately, this is now frowned upon as simple ventilation has been discovered to work effectively. I, personally, have tried just about everything known to the freediving world to try and extend my u/w breath hold time. This includes hyperventilation (before the dangers were really known and understood), inspiratory/expiratory muscle trainers, hypoxic training, alpha wave training, biofeedback, yogic meditation and packing. Currently, the emphasis appears to be in body alkalinity/acidity adjustment and metabolism. The folks over at deeperblue get pretty in depth on some of these topics. If I were you I would research some areas of interest and simply enjoy what you may find.
You mentioned that you want to stay underwater longer. It took me a very long time to realize that in trying to make myself stay underwater longer, I was defeating the purpose. Hard to convince people of this but instead of making yourself stay u/w longer you have to be patient and allow yourself to stay u/w longer. Yea, i know it doesn't make much sense does it?
There has been some good advice given here and remember that safety is paramount. By the way, if you want some good reading material, "Freedive" by Terry Maas is good as well as "The Manual of Freediving" by Humberto Pelizzari. Both are entertaining as well as informative and written by two of the elites in freediving.
