Try this. Use your PC muscles to continually contract your cock, one after the other. While doing that, use long up-strokes and pause at the top. What I do is pull the skin (I am cut) over the head and hold it there and contract my PC muscles. After a half dozen or so times of doing this, the moist dew begins to appear. That always gives motivation to keep doing what I am doing that produces a drop with each every third or forth up-stroke and then it starts flowing with each stroke. If you are not use to taking your time with self-pleasure it will take a while but once you learn it, it becomes easier and easier. Now, the wind can blow across my nipple and I start to precum.
A sure fire way to get the juice flowing is through direct stimulation. Take a look at File:Male anatomy.png - Wikipedia, the free encyclopedia and see that the glands (this pic is a cross-section, we have 2 of them, 1 on each side of the urethra), lie slightly below the prostate gland and kinda stick out, back toward the anus. Anything that stimulates that area will "hit" the Cowper's glands before the prostate as they are lower in the body. Many men will secrete fluid during a routine rectal prostate exam. You don't have to be hard to secrete it.
I would say that if prolonged external stimulation along with edging and some prolonged internal (doesn't have to be deep!) stimulation, does not produce the clear sticky fluid, you just might be one of the men that does not produce enough to be noticable. However, I did not think I produced it either because I was usually too much in a hurry to ejaculate or my cock was in a place where I would not see precum.
Give these a try but be patient with yourself if you don't see immediate results.
