Leeks can hold dirt between their layers so it pays to clean them carefully. The easiest way to get all the dirt out is to cut them before rinsing them.
First cut off the top of the leek where the color changes to a definite green. Keep the white and pale green part and discard the dark green top (or save it for stock).
Slice the leek in half lengthwise. Lay each half cut side down on the cutting board and cut across into half round slices. Put the slices in a collander and rinse under running water, tossing the leek slices with your hands as you rinse them.