When I worked at the Little Nell Hotel in Aspen, Colorado, executive chef George Mahaffey taught me this technique: Use warm water to refresh greens and then chill them in order to get them crisp. As with flowers, if the old stem of the greens is cut or torn to allow the water to be absorbed into the freshly cut end, the greens get crisper.
STEP ONE. Fill your sink with warm water. The temperature should be 105 degrees F to 110 degrees F; if it feels warm to your wrist, it is warm enough.
STEP TWO. Submerge lettuces, herbs or other tender greens and soak for 5 to 10 minutes.
STEP THREE. Carefully dry the greens, preferably using a salad spinner. Gently shake the greens to remove excess water before spinning. Don't overload the spinner or spin them too quickly, as the greens may bruise.
STEP FOUR. Cover and chill. Cover the greens with a damp towel, chill for at least 20 minutes, and you will once again have crisp, vibrant lettuces, herbs or greens.
Recipe courtesy Douglas Katz, Chef's Secrets, Chronicle Books, 2004