1. Training harder instead of smarter
Most amateur swimmers train consistently for weeks, months and even years and not see an improvement in their times. Most often, this happens because their goal in training is to swim further or harder than the previous week. Swimming is a unique sport because the biggest improvement comes from reducing drag and not increasing strength or stamina. Train to improve your technique, not your strength or endurance.
2. Stroking faster not longer
The common thinking among new swimmers is that to go faster, you must kick and pull faster. While this is partly true, the greatest improvement in speed will come from travelling further each stroke.
So if your travelling further for each stroke but still holding the same stroke rate, you'll be swimming faster with less effort.
3. Pulling through too early
Just like a batter who swings too early and miss hits the ball, a swimmer who pulls through too early will miss much of the water. Once your hand enters the water, allow it to stay high near the surface as you glide forward, and once you feel yourself losing forward momentum, begin your pull through
4. Looking forward and not down
Body position is the most important factor in swimming. It determines how much drag you produce, how quickly you can stroke and how well you glide. Most new swimmers begin by swimming freestyle with the head looking forward because it's the natural thing to do. For the body to be balanced in the water, the head needs to be looking towards the bottom of the pool. This will bring the hips and legs closer to the surface which reduces drag and allows for a smoother more efficient stroke.
5. Swimming flat and not rotating from side to side
Imagine a big flat barge moving through the water and how slow it travels because of the amount of resistance it creates. Compare that to a long and streamlined ocean liner with a V shaped hull which can travel for more than 2 weeks at over 34 knots (40 mph). Contrary to what many think, swimming is not performed on the stomach. It is done by rotating from side to side. This is how you reduce drag and generate speed and power. Look at Michael Phelps' freestyle and you will notice he rotates beginning with the hips and following through all the way to his shoulders (a.k.a shoulder roll).
6. Pulling through with a dropped elbow
The biggest weakness in 95% of swimmers stroke is the dropped elbow in the pull through. This is a result of weak shoulders and bad practice. A proper pull through will start with bringing the fingertips to point at the bottom of the pull while keeping the elbow high, and then pulling through using your entire forearm.
7. Leading the arm recovery with the hand
In all elite swimmers, you'll notice their arm recovery is relaxed and with a high elbow. This is acheived by leading with the elbow and not the hand, as most amateur swimmers will do. The key is to be relaxed, keep a high elbow and enter the hand early in the water, just past the head.