For the most part of my teen years, I used this technique to produce some of the biggest cums. If you do it right, the pounding water will literally milk your cock until your balls are dry.
I start by wrapping a hand towel around the head of the shower and secure it tightly by tucking it under. With the length of the towel hanging down, take the bottom end and bring it up and twist it again around the shower head. You want the water to have a way of pouring out in one big spout. And the higher that comes from, the harder the force of the water below. If you find it too strong, extend the towel so the water drops closer to the floor.
I lay down a towel in the shower stall and kneel facing the falling water. Using my open palm, I make a 'platform' so my penis can rest on top of it. Then I guide my cock so the stream of water drops directly onto it. Moving my hips occassionally, you can vary where the water hits your penis and get a variation of the feeling.
When I did this on a regular basis, I used to put some porn pictures up on the shower wall and go for it.
When you reach orgasm, the streaming water hitting at that powerful force will feel the same as if someone were milking your cock trying to get the very last drop of cum from your balls. The feeling was so intense that I often times with almost pass out as I watched rope after rope of sperm hit the wall of the shower.
After recovering, I just rinsed the wall and cleaned up. Enjoy...especially you guys that are beginning to masturbate. After trying thiis once, I'm certain you'll want to do this all the time. The only thing is if your home has a small water heater so you might take all of the hot water or where your parents would bug you for taking so long in the shower. What's great is when I went traveling in a hotel. You could shower for hours and not run out of water!