Make sure the lingerie is pretty and not trashy. Avoid China red, too many straps, too much sheerness, and garter belts. You want this gift to say she's a princess, not a prostitute. You can't really go wrong with soft pastels.
Material is very, very important.
Get bonus points and buy the matching robe. If you're not sure of her size, sneak into her privy drawer and check out her bra size. 36B is most common. Now that you're sure of the size and that the gift won't be returned, carefully cut the tags out of the item. Before you put the lingerie into the box, give it a misting of your or her favorite perfume.