Getting serious now...I have one technique that I have my wife do that puts me over the edge like nothing else as far as manual stimulation goes. First, it helps a lot to be very close to cumming, otherwise it could take a long time.

I lie back on my back and cover my erect cock with a shear cloth -- t-shirt or something. I then have my wife slowly move her finger across the hot spot right under the head on the underside (which is facing up since you're on ur back). I have her move the entire length of her finger slowly back and forth, like she's playing a violin and her finger's the bow. At first you might just feel pressure, but soon enuf there's an unmistakable sensation that just builds and builds and builds and builds. Having her "play the violin" at a steady, almost mechanical rate, is a must. The explosion is outta this world, for me at least.