Leeks grow into the soil and they retain lots of dirt in their layers and leaves. It is important to clean them thoroughly. Unfortunately just rinsing the leeks will not get to the dirt in the inner layers. I got this wonderful tips on how to clean leeks from Martha Stewart Living cookbook while preparing SHRIMP AND SCALLOP STEW WITH COUSCOUS PILAF.
The best way to ensure that you get all the dirt out is to follow these steps:
Cut the leeks into the size that you want. In general, half them lengthwise, then sliced crosswise into 1/4-inch thick pieces. Transfer the leek pieces to a large bowl of cold water, stir, and let stand for 5 minutes to let dirt and sand settle to the bottom. Lift leeks out of the water with a slotted spoon, and drain on paper towels