To prepare salad leaves, separate them from the stem if necessary and remove any discoloured patches. Gently swirl the leaves in cold water (or a mixture of water and white wine vinegar if you're concerned about bugs). Repeat as necessary. A salad spinner will help you dry them quickly and gently - your leaves should be thoroughly dry before use so that they do not dilute the dressing.
In general, it's best to tear salad leaves to the size preferred. If you're making a chopped or shredded salad, be sure to use a sharp knife to prevent the leaves from bruising. Bear in mind that the roots of some salad leaves such as corn salad, lamb's lettuce and mâché, are edible. To dress salads, pour the dressing into a salad bowl, add the leaves, then mix gently with your hands until the leaves are evenly coated.