1.  When the Temple was destroyed, the Sages in that generation decreed that one is not allowed to build a finished home like kings build unleess one left over an area one by one cubit by the opening that is unfinished without mortar or plaster.  If one buys a court yard that is already built and finished, then it has a presumed status (בחזקה) and one does not have to alter a portion of it. 

2.  They also decreed that when one sets a table to prepare a meal for guests, one should diminish from the meal a little. Furthermore, one should leave over an empty space that would normally hold another tasty dish that is being served at the meal {1}.  When a woman does cosmetic painting of gold and silver, she should leave over one of the cosmetics that she would normally use in order that it should not be a complete job.  And when a man marries a woman, he should take ash from the fireplace and put it on his head in the place of his Tefillin.  [There are places where we have the custom to break a cup at the time of the Chupah or to put out a black table cloth or to do other signs of mouning on the head of the groom (חתן).]  All these things are done in order to remember Jerusalem, as it says "If I will forget you, Oh Jerusalem ... if I will not ascend to Jerusalem on the peak of my gladdness".

3.  They also decreed that one may not play music with a musical instrament of any kind in order to get happy by listening to them.  [Others say that this only applies to those whe are accustomed to them (being played constantly) like Kings who get up and lie down with a musical instrament and it also applies during a meal.{2}]  And it is forbidden to listen to them because of the destruction.  If there is wine being consumed, then even a purely vocal song is forbidden to sing as it says "with song wine shall not be drunk".  It is already the custom of all of Israel to sing songs of praise or songs of thanksgiving and rememberances to the kindness of G-d on wine.  [Similarly, everything is permitted if used for a Mitzvah like at the house of the bride and the groom.]

4.  They also decreed that grooms may not wear any type of crown or wreath on their head as it says "the מצנפת is missing and the crown is gone".  They also decreed about the crown of the bride that it may not be made of silver.  But if it made out of fabric it is permitted for the bride to wear.  This decree only applies to the bride and the groom.  But for everyone else, men or women, they did not make this decree.

5.  It is forbidden for a man to excessively fill his mouth with laughter in this world.  {3}
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{1} M.B: It is possible that if one only does the first one it is fine since the reason that a space is left over is to remind people that a dish is missing that would otherwize be there.
{2} Such is our custom since we do have concerts.  We do not follow the strict approach of the Mechaber.
{3} Excessive levity leads to sin.  The way of the Gentiles is hedonism which leads to a wasteful existance and is not the path of the holy Jew who's goal is to emulate G-d and follow in his footsteps.  Happiness and rejoicing are righteous when used for productive purpoes like doing G-d's commandments and rejoicing in Hashem's glory.  The Mishnah Brura argues and states that even when performing the commandments excessive laughter can be harmful. 
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Shulchan Aruch Chapter 561:  The Law When One Sees the Cities of Judea, Jerusalem and the Holy Temple in Their Destruction (5 Paragraphs)
1.  One who sees the cities of Judea in their destruction says "Your holy cities they used to call you!" and one rents his garment (in morning) {1}.  [One only has to tear one's garment when one gets close to them like from the Scopos outside Jerusalem.] 

2.  One who sees Jerusalem in its destruction should say "Zion that they used to call you now lies in ruin!" {2}.  When one sees the Holy Temple in its destruction one says "Our holy house and our glory that our fathers used to walk in is burnt with fire and all of our glory is now in ruin!"  And then one tears his garment.  When does one have to tear one's garment (for Jerusalem)? From the Scopos.  Afterwards, when one sees the ruins of the Holy Temple, one tears his garment a second time.  All tearing must by one handbreath in size.  If one approaches the Holy Land via the desert since one will see the Holy Temple first, one tears a handbreath for the Holy Temple and subsequently when one reaches Jerusalem, one tears a second time any small amount.

3.  If one tears his garment for one of the cities of Judea, one does not need to do so again when one sees a different city of Judea.  This is with the exception of Jerusalem, where one must tear again.  If one rents his garment for Jerusalem, one does not need to tear again for the other cities of Judea. 

4.  The tearing must be done by hand and standing.  One rents his entire shirt until it reveals his heart {3}.  And one is never allowed to repair these torn garments.  However, one is allowed to stitch the fabric into a hem and use it for the children to wear and also to use as a patch for clothing.

5.  If one traveled to Jerusalem, then departed and returned less than 30 days apart, one does not tear again.  But if one returned after 30 days have passed, one tears again.  [The same applies for the other cities of Judea and the Holy Temple.]

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{1} The Mishna Brura writes that even if the city has Jews living in it, if it is ruled by Arabs it is called "in their destruction".
{2} MB writes that some authorities write that if one sees Jerusalem in its destruction may not eat meat or wine for that entire day.
{3} MB writes that one should tear on the left side since that is where one's heart lies. 
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