When I have a little time in the shower, I like to turn the water onto my cock, just long enough to enjoy a good erection.
After a few strokes of soap on my hand, I carry on with soaping and washing myself. With a quick stroke every now and then.
Keeping it hard is very easy, but stopping myself from going too far is much more difficult. Every now and then I do slip up a little and go over to a full ejaculation. And every time I promise myself to do it better next time. I keep practising.