Energy loss during swimming is effected by three main areas: the pace you swim at, your swimming technique and your fitness level.
I know what you're thinking: yeah, they are all very obvious. True but when they are all finely honed they work together to make you a very slick, efficient and successful swimmer. There is no magic pill or secret technique that will make you a better swimmer overnight. It will take a little practice, step by step.
Let’s focus on the most obvious one first: your butterfly swimming habit. Efficient butterfly stroke comes from a good body position and strong waves movement. Combine this with a powerful kick and long stretch over the water with the arms to make the whole stroke action longer and you will have a more efficient swimming stroke.
Your fitness level is the next most obvious and training and laps will increase and elevate your energy and recovery rate.
Butterfly is the most energy consuming of all swimming strokes because it requires the most frequently physical movement.
The pace you swim at will ultimately determine how much energy you will have provide. It is vital that you conserve some for your final length, ideally when your competitors will be running low on energy too. If you have more than the other swimmers in the race then you have the advantage.