We are in the nine-day period leading up to Tishah b'Av when we remember and mourn the destruction of Jerusalem 1,931 years ago. But as well as these annual rituals there are other commemorative practices that are mandated year round. Some of these have fallen into disuse. For example, the Shulchan Aruch states that we should not play or hear musical instruments until the Temple is rebuilt. Almost no one today keeps this (although there are religious communities in Jerusalem that have only a single drummer accompanying the dancing at weddings).
The permanent mourning practice that is most widely observed today is that of leaving a patch of wall in your house unpainted. The Talmud teaches that since the destruction, whenever one builds a new house, we should leave something about it, such as the plastering, incomplete. While God's house is in ruins, we do not make our houses perfect. These memorials have helped maintain the Jewish bond to Jerusalem for millennia.