1. When greeting a large gathering, or if you are in doubt that the person you greeted has heard you, it is reccommended to repeat the greeting 3 times but should not go beyond this.
2. It is the Sunnah to initiate the greeting and respond to them out loud/audibly
3. Make your greetings to all in a gathering, not just to those you know.
A man asked the Prophet (صلى الله عليه وسلم), "Which Islam is best?" and He (صلى الله عليه وسلم) replied: "Feeding others and saying As-Salaam both to those you know and you don't know" (Bukhari & Muslim)
4. One who arrives in a gathering should hasten to initiate the greeting.
5. It is sunnah for the rider to greet the pedestrian, the pedestrian to greet the seated, the few to greet the many and the young to greet the old.
6. When meeting, the better of the two is the one who initiates the greeting (Bukhari)
7. It is reccommended to greet young children.
8. When greeting those that are awake in the presence of those that are sleeping, one should lower the tone of the voice, such as not to disturb the sleeping, but be heard by those who are awake.
9. One should not "initiate" the greeting of peace to the people of the book.
10. When answering the Salam of the people of the book, one should reply "Wa-Alaikum" (And same upon you).
11. It is disliked to greet someone who is relieving himself.
12. When entering the masjid, one should first pray the 2 rakahs of Tahiyyatul Masjid before greeting those present.
13. When the Imam is delivering the Jumuah khutbah it is disliked to give salam
14. When meeting others, always begin with greetings of peace and then start your conversation.
15. Before one parts from a gathering it is sunnah to give the greetings of peace.
16. It is disliked to greet with the hand (like the people of the book).
17. When answering a letter one should also return the greetings.
18. Part of the full greeting is to shake your the hand. (brother to brother, sister to sister). [Al Adab Al Mufrad].