Leeks resemble a giant scallion, in that there are layers and layers that dirt and grit can get down into. First, slice off the roots, and the last couple inches of the dark green tops, and discard. Rinse the leek to remove visible dirt, then chop/slice into whatever size the recipe requires. Fill a large bowl with cold water, and add the chopped leeks to the bowl. Agitate the leeks a bit with your hand, and let them sit for a few minutes. The dirt/grit/sand will fall to the bottom of the bowl. Now remove the chopped leeks with a slotted spoon (or your hand), and place on a paper towel to dry.