Lettuce is not brown. Don't buy it if it is. Turn up your nose at bruises and other breaks as well. Veggies aren't usually fragrant. You should be able to pick up a whiff of their scent, but basically, a smelly head of lettuce is too old for your table.
Greens are firm and crispy.
If the leaves look limp, pass them up. Now that you've procured your perfect greens, take care of them. When you get them home, wash them right away and store them in the fridge wrapped in paper towels. You need to really rinse them well, especially at the roots. If they're going right into the salad bowl, be sure to dry them well or else your dressing will slide off.