Bear in mind when photographing an abandoned or ruined site that in most cases a viewer will never have visited the area. It is therefore the job of the photographer to set the site in context and lead the viewer in as though they were actually present. This can be achieved by showing the view through a doorway, hallway, corridor or arch in to the deeper interior.
Abandoned buildings have no source of power and are consequently illuminated by daylight. The roof may have collapsed, or the interior may be quite dark. Look for areas where natural light floods in to illuminate parts of rooms or corridors and allow detail in darker areas to fade where the contrast is too great. Capturing the underlying atmosphere is what really counts.
Use a tripod and very long exposures to maximize depth of field where necessary, since it is unlikely that anything will be moving substantially. Artificial light, such as flash, can be used to reduce contrast but should be used sparingly to avoid ruining the atmosphere of decay and negligence. Where shadows are filled, make sure they are not over-filled. More images of this type are ruined rather than saved by flash.