I have been masturbating since I was about 12 years old. This technique was my very first that I tried.
When in the shower, once you use something like shampoo that makes you all slippery and smooth, hold your dick with your left hand and put the palm of your right hand on the tip of it. Start rubbing in a circular motion slowly, and when it starts getting more sensitive, speed up a bit. This method takes a while to make you cum, but it feels very, VERY good.