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.
Breathing is a crucial part of swimming. Everyone knows this, but so many swimmers struggle to get it right. They often opt to lifting their heads forward or just keep their heads out of the water the entire time. These techniques not only make you less efficient in the water but can actually make your overall breathing significantly harder. Developing good breathing technique is one of the hardest thing for new or intermediate swimmers to master.
Body position is crucial to efficient swimming. It has the ability to affect your endurance and speed, especially as your body begins to fatigue. Body position is a key foundational element that coaches should cover early and often with their athletes. No matter how many years you've been swimming, you can always continue to work on and improve your body position and roll.