To wash salad greens, start by filling a clean and sanitized kitchen sink with cold water and a little ice. The purpose of the ice isn't to create an ice bath, but rather to insure the water stays cool during the washing process.
Next, gently agitate the greens dunking in your water bath, removing any surface dirt. Here I'm using hydroponic butter leaf lettuce which still has the root attached. Specifically for a head of butter lettuce, hold the head by its root end and gently plunge up and down. Keeping the head of lettuce whole with the root attached will keep it "alive," extending the shelf life when properly stored.
If I was washing loose, leafy greens such as you would find in a mesculin or spring mix, I would simply submerge the greens in cold water and slightly agitate until all surface dirt was removed.
Once thoroughly cleaned, remove greens from water and place in a salad spinner. In the picture below I'm using a large commercial version that has a drain whole on the side. Smaller, home models usually just catch excess water in the bottom of the salad spinner which is then poured off after the spinning process. Insuring salad greens are properly dried is an important step in the washing process. Since vinaigrettes contain oil, any residual water left on the green's surface will repel the dressing, causing it to pool at the bottom of the plate instead of clinging to the salad.
After the greens are spun dry, remove to an appropriate sized storage container lined with slightly damp paper towels. In the case of the whole heads butter lettuce, I like to store root side down. Cover the top of the greens with damp paper towels and store in your refrigerator. When stored properly, loose, leafy greens can last up to four days, and hydroponic butter lettuce with the root attached will last about a week if treated kindly.
Quick Note: It should be made clear that this process will just remove surface dirt and not bacteria or viruses from the salad greens. Bacteria is most commonly killed by either chemical sanitizers or heat, neither of which are appropriate for raw salad greens. It is always advisable to purchase all of your food items from reputable suppliers or companies, especially those which can not be sanitized or heat treated.