1. Kick More - or Kick Less
This is really three tips. If you are not doing isolated kick training in a workout, start. Use a kickboard, don't use a kickboard, that is up to you, but add some sets where all you do is kick, no arms.
If you kick a lot when you are swimming, try kicking less forcefully, with less effort, or kick less per stroke. Put more emphasis on shoulder and hip rotation and on what your arms are doing and less on using your feet like a fish tail.
If you don't kick much when swimming, try kicking more sometimes. Do you find that doing this speeds up your swim? I am not an advocate of over-kicking for most folks, but the faster you want to go, the more a good, fast kick will come into the equation.
2. Swim More Often
How often do you swim? Less than three times each week and you are probably not doing enough to improve. I believe that most swimmers need to get splashing at least three times each week, and swimming four or five times each week is even better. The way I see it, three 30-minute workouts is better than two 45-minute workouts in a week.
3. Slow Down - or Speed Up
If you go as fast as you can from the time you start your workout to the end of your workout, you are not getting the full benefit of that workout. Start the workout at a slower pace, warm-up and prepare your body for the workout (see the next tip, too). Do some swimming at fast efforts and some at moderate efforts; mix up the speeds and/or efforts of your workout and you will get better.
One example: after warming up, swim fast for a short distance and then rest, then repeat. Do this several times in a workout over 3-4 weeks and you will swim faster (swim 4 to 8 x 25-meter efforts at a very high speed, with rest between each 25-meter swim equal to 3x or 4x the time it took you to do the 25-meter swim).
4. Do Swim Drills
I cannot state this one swimming tip enough. Do some kind of stroke technique-focused work every time you do a swim practice. Drills, drills, drills. Do some early, so some in the middle, and do some at the end. They will never hurt and can always help make you better by reminding you of some aspect of good swimming.
5. Try Different Breathing Patterns
How often do you breath when you are swimming? Mix it up. Try breathing less often or more often. Work on bi-lateral breathing - breathing on both sides - to help balance your stroke. Some swimmers take a breath every stroke cycle, some every 1.5 cycles, some less often, even as little as one breath every length of the pool (that is not enough breathing if you are going to be swimming more than a fast 50!).
I like breathing every stroke, but alternating sides by length of the pool; at an outdoor pool, if the sun is coming up on the East side of the pool (or if the pace clock is on the West), I will only breath on the West side of the pool; that changes which side of my body I breath on each length.
6a. Point Your Thumbs
This is number 6 - a bonus tip! This is a little one that might help some swimmers. As soon as your hand enters the water in freestyle (front crawl) try to aim your thumb toward the place you want to go. This can help set-up a better pull pattern in may swimmers.
6b. Point Your Middle Fingers
This is number 6b - a double bonus tip! Instead of trying to point your thumbs, try pointing your middle fingers (all of your fingers together, but use the middle as the focus) toward your destination, and then keep that finger oriented on a line that runs perpendicular to your destination, like your finger is on a railroad track that runs straight ahead, no left or right curves; it might go up and down, but nothing sideways.