First, the 3 rules of breath-holding in our club (mainly for kids):
- never do breath-holding alone
- never do it static
- never try to beat the world record (no search for blackouts)

We do the following exercises:
- warm-up: swim 12.5 surface and 12.5 underwater, this is to introduce breath-hodling exercises
- the hunter way: as another warm-up, players have to manage their time to cross the 25m pool. Give them 60 sec to cross, then give another start at 55, 50, 45, 40, 35 and 30. They can swim as fast as they want and use that extra time to recover, but recovery time is gradually decreasing
- the cleaner: go to the bottom of the pool and find some dirt (there always some). With your hands, try to make this little dust move forward without touching it (you need to do some retro-paddling with your hands). You can come to surface when you want, but the goal is to make it travel the 25m. If you put 5 kids in a row and start a competition, you'll see their performance improving. As a variant, use a coin.
- push him: take 2 players of the same size and tell them to stand on the surface holding the other guys shoulder. They have to come down to the bottom and push the other guy to make him go backwards. You really have to push hard and stick to the bottom to win this game
- stop & go: (my favorite) leave some pucks at 10, 15 and 20 m on the bottom. Players have to start at the wall and kick as fast as possible to the first puck. There, they have to wait, standing still for 3 seconds. Then, kick fast to the second puck and wait again. Finally, kick fast to the last puck and wait the last 3 seconds before reaching the last wall. Gradually make it harder by increasing the time to stay. Our local record is 8 sec, I think
- the counter: players have to cross the 25m pool underwater as much as they can in a 5 min timeframe. They can take as much rest as they want but then they can do less laps ...