The Chida (Rav Chayim Yosef David Azulai, 1724-1806) mentions that when one visits a grave, before taking leave he should place his left hand on the grave and recite the following verse from the Book of Yeshayahu (26:19): "Yechayu Meitecha, Neveilati Yekumun; Hakitzu Ve'ranenu Shochenei Afar Ki Tal Orot Talecha Ve'eretz Refa'im Tapil" ("Your deceased shall live, let my [nation's] corpses rise; awaken and exult, those who lie in the earth, for your dew [that will restore life to you] is like the dew [that descends upon] fresh earth, and the ground shall spew forth its corpses"). This verse is recited as a prayer that the deceased shall have his life restored at the time of Techiyat Ha'meitim (resurrection). It contains fifteen words, corresponding to the fifteen joints on the human hand three in each finger. The placement of specifically the left hand upon the grave is in accordance with the teachings of Kabbalah.
Rabbi Yehuda Ha'chasid (Germany, 12th-13th century), in his work Sefer Chasidim, writes (listen to audio for precise citation) that one should not visit the same grave twice in a single day; rather, one should recite all prayers and express all his requests before taking leave initially. Once a person takes leave of the grave, he should not return to visit the grave until the following day.
Summary: One who visits a grave should, before leaving, place his left hand on the grave and recite the verse in Yeshayahu 26:19. After leaving the grave, one should not return to the grave until the following day.
