What are the rulings of visitng graves and graves of pious people?

I am looking into the rulings established of what is permissible to do and not to do when visiting a grave in general, and visiting graves of prophets and pious people. This question was spurred from discussions in this question . I tried to search my best, but I really found it rather difficult. Many sources I found of different schools of Fiqh tells that it is okay to visit a grave, but there are certain innovations that should not be done, but they don't go on telling what are they. I am looking for the views of four schools of Fiqh, as to draw a general guideline.