A soggy pile of wilted greens makes for a sorry salad indeed. Tender greens like Boston lettuce, mâche, and arugula are delicate little things that perish at the mere rumor of mistreatment (tearing or roughly handling lettuce bruises it), but even crisp, hearty lettuces like romaine need to be treated with care. To keep them at their best, you need to consider three factors: time, volume, and temperature.
Only dress your greens just before serving, particularly when using vinaigrette: Oil quickly permeates the waxy surface of leafy greens, turning them dark green and droopy. If you've washed your greens, use a salad spinner or blot them delicately with paper towels to dry them. Water clinging to leaves will repel oil-based vinaigrettes and thin out creamy dressings, leading to bland salad.
Put dry greens in a salad bowl. Add less dressing than you think you'll need (to avoid overdressing), and pour it down the sides of the bowl, not onto the greens—you'll dress them more evenly this way. Gently toss, adding dressing as needed, until the greens are lightly coated. If you do overdress them, a quick whirl in the salad spinner will shake off any excess.
Finally, follow the lead of professional chefs and serve your salad on chilled plates to help keep the greens crisp as you enjoy them.