There is one technique that I rely on more than anything else when teaching swimmers the mechanics of swimming. It's sculling. Most sessions will have at least a few lengths of sculling, whether it's part of a warm-up, cool down or even part of the main set. Learning how your body moves, manipulates, and feels in the water is a crucial part of increasing both your speed and efficiency in the water.
There is a misconception about the proper breathing for Butterfly. Many swimmers tend to come out of the water as much as they can when breathing in fly. This causes their hips to sink in the water meaning they have to expend additional energy to drive their hips back up to the surface. The result is usually a short pause between each stroke. That short pause, even if it's just for a split second, can throw off your entire rhythm.