Press Lung: Ideal buoyancy in the water is achieved when the lungs, your body's natural life preservers, are pressed down towards the bottom of the pool. This is the foundation of downhill swimming and the position that allows for a more streamlined body. Think of your body as a teeter-totter, with the hips as the fulcrum. Pressing the lungs down brings the legs up, and vice versa. If the legs are down, they simply act as anchors, producing drag against the water that slows the rest of the body. Whether in the front or side swimming positions, always focus on pressing the lungs down towards the bottom of the pool. Once this becomes natural, you'll conserve enormous amounts of energy and see massive increases in speed.
Brush Thumb: A proper and full stroke should bring the thumb to brush against the thigh at the end of the pull phase. Too many swimmers cut their pull short, for the simple reason that it makes swimming easier. I guarantee that if you practice a full pull phase, you will feel horrible during your first few swims and the muscles will be completely fatigued by the end of the swim. After practicing for a few weeks, however, your body will adapt and your speed will skyrocket. One of the keys is to achieve the thumb against thigh position by using the powerful lat muscles underneath the armpits, not the relatively weaker biceps and forearm.
Boil Feet: The feet should be "boiling", just below the surface of the water. Feet that are submerged far below the surface are simply acting as drag-producing anchors, while feet that kick and splash above the surface are wasting too much time kicking in the air. We all know that the air produces no resistance, so this is wasted energy. Think about making tiny bubbles with the feet as you kick. While triathletes should not be wasting precious muscle glycogen stores in the legs during the swim portion of the race, a low-medium effort kick will be enough to keep those foot-anchors up.
Hide Head: If you are in a proper downhill swimming position, just a sliver of the head will show against the water. As you practice "Press Lung", a natural consequence should be that the head "hides" below the water. If your head/torso unit is high, your feet will drop. Once again, buoyancy is a crucial key to efficient swimming.
Puppet Elbow: Imagine that your elbow is attached to a puppet string that is pulling it straight out of the water in the recovery phase of the stroke. A full elbow recovery is very important, especially in choppy, open-water swimming, where a partially submerged arm in the recovery phase will quickly tire you out because of increased drag.
Cigar Mouth: For a streamlined breathing pattern, attempt to take as little of the head as possible out of the water when breathing. The best way to think about this is "smoking a cigar" when you inhale, meaning, for you healthy, non-smoking triathletes, that the breath only comes from the outside corner of the mouth while the inside corner of the mouth is under the water. As you learn this breathing method, you may end up swallowing a bit of water, but long term practice will result in more efficient swimming.
Raise Pinky: To achieve optimum pull against the water with the hand, while still maintaining a drag-free slice through the water, the pinky should be elevated higher than the rest of the fingers during the entire stroke phase. Every hand is different, so experiment with the outwardly turned angle of the hand until you find a position that gives you the most speed. One of the common mistakes I see when the pinky is elevated is a completely locked out elbow. Never completely straighten the arm when reaching towards the end of the pool because you'll be able to grab less water to pull against.
Wall Reach: "Reach Over a Wall", "Spear a Fish", "Take a Cookie From the Jar" - there are many ways to describe how your hand should feel as it enters the water, but the general idea is that you are grabbing as big a handful of water as possible when initiating the pull phase of the stroke. If your elbow was correctly drawn out of the water, this will result in a more vertical entry of the hand/forearm unit. Remember, the forearm creates pull against the water in the same way as the hand, so make sure to use it by keeping the elbow slightly bent as you reach over the wall.