I've always scoffed at visualisation techniques, especially one recommended to me for childbirth, where an ocean was flowing into and out of a cave. In the end that didn't relate at all to my c-section
BUT. I have found that closing my eyes while DS is furiously suckling away, and breathing out while simultaneously thinking "downward" thoughts, almost physically settling my shoulders down, and imagining the milk starting to flow, will most times hurry the familiar stinging sensation along.
Sometimes I don't feel let down at all, but DS still seems to be quite content, and stays on for the same length of time. It happens mostly in the am when my boobs are fuller, kwim? Does your LO seems to be having any issues, or is it just that you're not feeling the let down?
Katy