If you're asking this question then you're looking for some tips on how to swim as fast as Micheal Phelps racing a Shark.
To delve right in, the best way to improve speed and reduce exhaustion is efficiency. Efficiency, efficiency, and efficiency - ok hopefully that's drilled into you now. So what do I mean by efficiency, well that's the tricky bit? Let me give you a few examples of how you need to think about your body when your swimming that might change your perception of it.
Look at a container ship, they are massive, sturdy, powerful. They have a wide bow and move pretty slowly through the water, well wouldn;t we all with that much resistance against us? Just like a speedboat sits above the water, just how racing boats have really thin hulls. Your body needs provide as little forward resistance as possible - this means being on your side a lot of the time.
I'm not going to write loads about this so here's a link to a really useful video showing you how to swim on your side to provide little resistance.
Use your legs for balance - not propulsion
So I'm going to sit on the couch here and say I grew up barely training my legs in training and paid for it a lot when it came to sprint endurance. One thing I do know though is that when you are wanting to swim in cruise mode, infrequently using your kick is the best way. You should only be using your legs to maintain body position as you are rotating on your side.
Long is strong.
Another top tip, keep a long stroke. The more you can glide on one stroke the less effort you are putting in, if someone takes 30 strokes to do 50m and another person does 40, I bet you'll find the second person slowing down faster after a while.
Counting your strokes is a very good way of gauging your swimming efficiency - you can quickly notice when your getting tired that maintaining the same amount of strokes per length or distance becomes increasingly more difficult.
Don't lift your head.
So this one is specifically for freestyle or front crawl swimmers. If you imagine you are a plank of wood (no offence) floating in the water. If someone lifts up one end, the other end sinks. Simple. That's pretty much the issue if you lift your head upwards to breath while breathing to the side on front crawl, you should always try to breathe in the space created under your armpit by rotating your head as if it had a pole running all the way up your back attached to it.
The effect of lifting your head means your body's forward resistance is much greater because you have the distance from your head all the way to your sinking legs trying to push through the water, rather than just your head and shoulders punching a hole for the rest of your body to swim through.
Spread your fingers out.
It kind of seems counter-intuitive to some people but when swimming front crawl you should loosen your hands a little so your fingers have a bit of a gap between them. Although it would seem this is less streamlined and would let more water through, in the past decade it is has been recognised that swimming this way actually increases the surface area of your hands and gives you a better 'grip' on the water.
It's also a lot more comfortable to swim this way! No more trying desperately to squeeze your fingers together in the neatest shape possible so they can enter the water perfectly.
Your core muscles are the key.
Well, this one is a bit boring and maybe very obvious but you know got to have it in there just as a side note. Swimming is mainly about core strength when it comes to muscles and muscular endurance. Your floating and balancing all the time and rotating your hips constantly. If you have a strong core, all these things can be handled easily. If however, you have poor core strength, the fundamentals of your swimming will always be difficult to master effectively.
Flexibility in swimming is essential.
So all this talk of long strokes, rotating your body and maintaining positions in the water all rely on great flexibility. Especially flexibility in the shoulders, ankles and hips. If you have great flexibility in your shoulders, your muscles don't have to work as hard and therefore don't get as tired. You'll also be able to maintain longer strokes and therefore be more efficient in the water. You can have all the muscle in the world but without flexibility it's pretty useless, to be honest.
Hopefully, some of these points have at least been useful. Even if you can master one of these you will improve your swimming speed and be able to swim faster without getting tired.