It may not always be possible to rinse and air-dry produce a day or two in advance, but if you can, great. Placing them in the refrigerator makes lettuce crispier, herbs firmer, and in my opinion it yields crunchier salads. Various vegetables are rinsed differently.
Place a large, clean towel on the counter before you begin. Bunches of vegetables such as parsley, mint, oregano, cilantro, or dill should be plunged into a bowl of cold water to rinse out deeply embedded debris and sand. Change the water several times until it runs clear. Shake out as much of the excess moisture as possible from the bunches. You can use a salad spinner, if you have one, to remove excess moisture.