The biggest rule of screenplay writing is that if you cannot see it do not put it in the script. A screenplay is a blueprint for the screen and if you cannot visually describe what you are writing in the script then there is no way for the director or actor to create it visually.
A common mistake is to put the character's thoughts in the action lines. How can we see his/her thoughts?
"Show, do not tell" is another great rule regarding what to include in a script.
Page numbers on the top right of every page.
ALL CAPS for the first introduction of a character in the scene description only. Make sure and give us a description of him/her close to when we first meet the character so we can have a clear picture of him early on. By putting the character name in all caps only at the first introduction, the reader will know that when character appears later and is not in all caps that we have already been introduced to him/her.
Please translate any language other than English in the screenplay. One common way of doing this is to put a note in the beginning of the script that the entire script will be in Afrikaans or Zulu and then to have English as the dialogue spoken for the reader.
A great document the Academy for Motion Picture Arts & Sciences created for describing all the different elements of a screenplay with regards to formatting can be downloaded below.