After years of dedicated research and practice, I recently managed to push the boundaries of my solo experience. I don't know whether it is through technique or physical change, but it is certainly interesting and means that I often ejaculate twice in a few seconds when before I did it once.
This phenomenon uses muscle tension as a source of stimulation, which I think is part of everyones experience. For example, if you're playing with your cock lying on your back, it probably makes you feel hotter if your legs are held straight not bent.
Usually when I'm masturbating I lay on my back or sit on the floor, shoulders on a wall. If then I bend my legs with feet close together, there is pleasurable tension in my inner thighs and cock. I stretch the muscles in my thighs by using my hands to push my knees down or by putting pressure on my inner thighs directly, making the tendons more taught, like putting your fingers on guitar strings. This has a very nice effect of taking me closer to orgasm without me touching my cock. It can be used as a kind of fine adjustment, at a time when touching my cock would make me come everywhere. It takes a bit of concentration but by 'fine tuning' I can bring myself right to the brink of coming, apparently close enough to make my balls squirt most of their load but not enough to have an orgasm. I always thought the two were the same, but apparently not.
It is very enjoyable to feel the come travel along the tubes and see it do it's stuff without being in the throws of an actual orgasm. With some of the pressure released and arousal still high I can masturbate a while longer. Usually the experience is such a turn-on that I go onto a full orgasm quite soon! The second ejaculation is a proper orgasm but with half the volume of semen.
I can only do this after taking myself near to coming and then back for at least an hour so that my balls are just acheing to squirt, but I hold them back.