Butterfly is the most physically demanding stroke for most people to swim. It is far easier to swim the other three strokes legally when you are tired. Fly requires strong shoulders, lower back, and leg strength.

I have been swimming Masters for five years and refuse to race 100 fly! I was a good butterfly swimmer in high school but I have poor fly endurance so I concentrate on the 50 fly only. But if you want to swim 100 fly I suggest the following:

1. Swim only good fly in practice. I don’t think it is useful to swim broken down fly because your mechanics and timing are all wrong.
2. One armed fly is a good standard drill that reinforces the proper timing of two kicks per arm stroke. I prefer this drill with one arm in front, not at the side. Variations include 3 arm strokes on each side; 3 arm strokes left, 3 right, then 3 both arms. Make sure that your one-armed fly still resembles butterfly, not freestyle.
3. I think fly kick without a kickboard is superior to kicking with a board. You will be in a more natural position.
4. Swim 25s or 50s of good fly as you work your way up to a 100. Periodically swim some fast 25s so you develop the race-level timing, which is often very different from practice fly.
5. You can use fins to help you swim longer distances. But don’t overdo it with fins. You have to develop the kick to allow your arms to recover cleanly over the water.
6. I wouldn’t try to starve your lungs. Breathe when you need to and use the breathing to develop consistent rhythm.