Paint the closet door the same color as the rest of the walls. This includes the trim around the door and any hardware on the door, like a metal handle for gripping.
Arrange eye catching things on the opposite side of the room. Place a room focal point -- like a television, a brightly colored, dramatic painting, or even a high-contrast colored accent wall -- on the opposite side from the door.
Place furniture in the visual path of the sliding closet door so that you can't see the area of the room with the closet when you first walk in. For example, place a tall bookshelf or a tall plant near the entry way of the room, in a position so that when someone enters the room, the bookshelf is visually blocking the area of the room where the sliding closet door is located. This strategy will not work in many rooms, because the placement of the furniture or plant should be subtle enough to avoid looking strange or out of place.
Place a sofa or reclining chair directly in front of the sliding closet door. Leave just enough space -- maybe 18 inches -- so that you can squeeze behind the sofa or chair in order to slide the door open and access the contents of the closet.
Hang a curtain rod over the door and place a decorative curtain or carpet on the rod.