Sunan of Greeting, Meeting, and Gathering

1) Greeting others with Salām (The greeting of peace).

There are numerous narrations that indicate this Sunnah.

Replying to the greeting of Salām is obligatory.
Allāh, the Almighty, says, “And when you are greeted with a greeting, answer with a better greeting, or at least return it equally. Verily, Allāh keeps account indeed of all things.” (Soorat Un-Nisā’)
An order from Allāh or the Prophet (PBUH) originally means it is for obligation, unless another authentic proof proves otherwise.
Agreement of all scholars on the obligation of replying to a Muslim’s greeting has been reported by several scholars, some of which are Ibn Hazm, Ibn ‘Abdil Barr, Shaykh Taqiy Ud-Deen, and others, may Allāh have mercy on them.
The best formula of the greeting of Salām and response to it is, “May peace, the mercy of Allāh, and the blessings of Allāh be upon you.” 
Ibn Ul-Qayyim, may Allāh have mercy on him, said, “The Prophet’s Sunnah when greeting someone is to complete this formula to the very end where it says, ‘... wa Barakātuh.’ (i.e. and the blessings of Allāh be upon you).” (Zād Al-Ma‘ād 2/417)

2) Repeating Salām three times if needed.
This is only valid in some cases, like when there is doubt whether the other person heard your greeting or not, in which case it is recommended to repeat it up to three times. Another example of this would be when there is a big gathering and only those in close vicinity hear the greeting, so repeating it is recommended up to three times in order to have all people hear.
This is proven by the following Hadeeth:

It can be concluded from the previous Hadeeth of Anass (ABPWH) , that it is Sunnah to repeat a word twice or thrice if needed, like in a situation where a person talks and he is not heard or understood.

3) Greeting those whom you know and those whom you do not know.

4) Initiating Salām according to the rules that the Sunnah illustrated.

There is no problem, however, if someone does the opposite, like if the old greeted the young or if the one walking greeted the one riding, but it is of course better to follow the rules mentioned above as illustrated in the Sunnah.
5) Greeting children and shaking hands with them.

Greeting children and shaking hands with them shows humbleness, gets the children used to this, and raises the value of the Sunnah in their hearts.
6) Greeting those at home upon entering home.
This falls under the general category of greeting others with Salām. It is Sunnah upon entering one’s home to begin by cleaning his teeth with Siwāk (tooth-stick), and then to greet those at home with Salām.

Thus, he used to use the Siwāk first, and then greet his family members.
Some scholars even said that it is Sunnah to say the greeting of Salām upon entering any house, not just one’s home, and that this should be done even if there was no one inside the house, as Allāh the Almighty says,

From the explanation above, we can conclude that there are three Sunan upon entering home:

2) To use Siwāk, according to the Hadeeth of ‘Ā’ishah, may Allāh be pleased with her, that we already mentioned.

3) To greet the members of the family who are at home upon entering home.
7) Lowering one’s voice when greeting people if some of them are asleep.

8) Conveying the greeting of Salām to others when asked by someone to do so.
Conveying Salām is Sunnah when asked by someone to convey Salām to someone else.

This Hadeeth indicates that one who is asked to convey greetings should convey them, like the Prophet conveyed the greeting of Jibreel to ‘Ā’ishah. Moreover, the Hadeeth also indicates that it is Sunnah to send greetings to others with someone.
9) Greeting people with Salām upon arriving at a meeting, and upon leaving.

10) Shaking hands and greeting others with Salām upon meeting each other.
This was the practice of the companions who were very keen to follow the Sunnah.

11) Smiling.

12) Using polite and decent words.
Using polite and decent words when meeting or sitting with someone is from the Sunnah, and is an act of charity.

Polite and decent words that people commonly use can be a means to obtain great reward, if just accompanied with the intention of doing charity.
Shaykh Ubn ‘Uthaymeen, may Allāh have mercy on him, said, “Pleasant words are such as saying, ‘How are you?’, ‘How are your brothers?’, ‘How is your family?’, and so on. Such words have an effect of pleasure on the heart of the person to whom they are said. Every pleasant word is a charity and its reward is bestowed by Allāh the Almighty.” (Sharh Riyādh Is-Sāliheen, 2/996, Chapter, “Recommendation of pleasant speech and meeting people with a cheerful face”)

13) Remembering Allāh in meetings and mentioning His name (i.e. praising Him).

14) Concluding meetings with “Kaffārat Il-Majlis” (i.e. Expiation of meetings).