Total Immersion has this drill called the Superman Glide. You basically push off the bottom of the pool (not the side; that’s cheating!) and launch yourself forward into the “Superman” flying position.
The object is to practice good streamlining, relaxation and no tension. If you do it right, you will zip farther in the pool before you stop than if you have poor streamlining or have an extremely tightness in your body.
My Coach Shinji can glide 18y on a single push off. It’s pretty freakin’ amazing:

One exercise I did was to see how many Superman Glides it would take to get to the end of a 25y pool. At first, it took me about 4.5. Over the last few months, I experimented with a lot of tweaking of my body positioning and finally made it in 3 glides. Some things I found that worked:
1. Relaxing is much better than stiff or tight. I just exercise enough tension to hold my arms straight out in front of my head, and to extend my legs. But no more than that.
2. However, relaxing totally didn’t work either. It meant that my body was a bit too loose and resulted in a less streamlined profile than holding enough tension to extend my body more sleekly.
3. I discovered that narrowing my body profile by extending my arms forward of my head is better than just putting them out there in a “V”. my shoulders are actually extended forward with my arms so it reduces the width of my shoulders.
4. I straightened my back, which feels a bit like arching the back to remove the natural curve of the spine. This is also achieved by rotating the hips forward a bit. A flat back seems to make me go further.
5. My legs do not just hang back relaxed. There is too much drag if they are just hanging out straight. Instead, I make a conscious effort to keep them straight back and touch my feet gently together, which puts my legs in my slipstream.
6. On the push off from the bottom of the pool, I plant my feet firmly before pushing off. I also try to push forward, which is tough because the bottom of the pool is slippery. It is better to plant the feet on the non-tile portion of a lane. Tiles are much more slippery than the other non-tile surfaces of the pool bottom.
I push off as straight as possible. Any angle or upset in my direction will either push me into a lane line or cause me to rock, which increases drag.
I try to push off as hard as possible. This is very hard because my feet slip on the bottom, even if they are not on the tiles. A harder push means more forward momentum, but is hard to achieve because your feet don’t have a nice surface to grip onto.
7. As I glide and slow, there is a tendency for my legs to drop. I try to flex whatever muscles in my back I can to keep my legs as high as possible, and to extend the time before my legs drop. Letting your legs drop is OK as far as the exercise goes, but it does not let you achieve your maximum length glide.
I only let my legs drop when I come to a complete stop.
8. By the way, you should be rested and not out of breath from warming up, or doing laps before. Gliding a long time also means holding your breath a long time until your forward motion stops so you don’t want to stop the glide early just because you’re running out of air!
9. I am very sensitive to the water flowing around every inch of my body. As I develop my position, I try to sense where is water causing drag on my body and where it is not. If there is drag, I try to change something on the next glide to see if I can remove the drag. This is also helpful during regular swimming, which is to see if anything on your body is slowing you down.
One of these days, I hope to achieve an 18y ultimate Superman Glide!