Your goal is to capture the romance and nostalgic feeling that is often associated with these former structures and not so much on their tourist appeal since most major tourist ruins have been photographed to death.
Photograph your subjects from various angles, perspectives and by day and by night. Using flash will be required at night and a diffuser will probably be handy during the day. It is always better to photograph when you have a diffused ambient light condition such as when the sky is overcast.
If you take a photograph of a rock structure upon which there are some reliefs during the midday sun and when the sun is about to fade or even at dawn you will see much more detail and the reliefs will be easier to admire during dawn,dusk or when overcast.
Midday sun light creates a "washed out" effect and most details will be hard to distinguish not to mention a noticeable lack of visible texture. This is why photographing while the light is diffused makes sense to most photographers.
Also, don’t just focus of capturing images of the subject’s exteriors include aspects of the interiors whenever possible and safe to do. Take long shots, wide angles, close ups and include elements of the surrounding scenery to put into perspectives the location where your subject is located.
A lot of photographers like to include the human element in their ruins photography, I do not since if I wanted to take photos of people I would do it under another theme. I just want to concentrate on the ruins themselves and allow my viewers to immerse themselves in the images before their eyes. This will often lead them to imagine the history of the place without the interference of modern details.
A good staring point to ask yourself is "what photograph can I take that will let others feel as I felt ". Seek more than ordinary pictures. Instead capture photographs that invoke a feeling instead of photographs that just look pretty.