Any wardrobe rail you are considering should be heavy-duty enough to hold a significant amount of clothing as well as the weight of the hangers from which the clothing will hang. You will need to measure your wardrobe to find out what length of wardrobe rail you need, and while adjustable rails are available, it is usually best to find a rail that will fit your rail without telescoping too much. Choose from metal, wood, and even plastic rails by determining how much weight will be hanging from the rail, and what kind of aesthetics you want to achieve.
Perhaps the most important part of any wardrobe rail is the set of brackets used to mount the rail. These are usually mounted on either end of the rail, and they can be mounted with screws to the sides of the wardrobe. These brackets should hold the rail in position so it is sturdy; there should be no excess movement, and the rail should not be able to slide out of the brackets. Some brackets are suspended from the ceiling of the wardrobe rather than from the walls; the choice of design is largely a matter of preference, based on ease of installation as well as space savings.
The rail itself does not need to be made from exceptionally heavy-duty materials unless a significant amount of weight will be hanging from the wardrobe rail. Many rails are made from aluminum because it is lightweight and inexpensive, and such a rail should be fine for most wardrobe purposes. If heavier garments will be hung from the wardrobe rail, or if an aluminum rail will not fit the aesthetic of the wardrobe, other metals such as steel can be used, as can wooden rails.
Wooden rails are perhaps the most attractive choice for wardrobe purposes. The wood used to make the rail can vary according to the materials used for the rest of the wardrobe. Installation of a wooden rail may be a bit more difficult than that of a metal rail, but the difference will not be too significant. Choose a wood that fits the color scheme of your wardrobe, and avoid softer woods or woods prone to warping. Pine can be used, but it should be treated to prevent warping, cracking, splitting, and other common problems associated with pine. A cedar rail is a good choice because it is resistant to such damage, it is naturally beautiful, and it even has properties that help prevent bug infestations.