Use the correct paper: The paper you use must be 8- 1/2 x 11 inch white 20# bond, and the screenplay must be printed on one side in black ink only. This is the industry standard and should be followed exactly.
Use the correct font. Courier New 12 point or the Courier Final Draft font should be used. The margins should be 1 inch around all sides of each page. The length of a movie can be determined by the length of the screenplay. Therefore, if you mess with different fonts, it will be harder for the reader to determine just how long your screenplay will run.
Have the right cover. You should use card stock that is either white or beige for both the front and back cover of your script. A script gets handled multiple times, so the card stock is necessary to protect the screenplay. The screenplay should also be held together by three brass brads. This makes it easier to take apart the script for photocopying.
Put your information on the cover page. The title, author and your contact information should appear on a separate cover page and no where else.
Pay attention to the rules. If the studio or agency you are sending your screenplay has specific guidelines for submission, make sure you follow them to the letter. They come up with these rules for a reason and no matter if these rules seem trivial to you, they can mean the difference between your screenplay being read or skipped over.
Revise after spell-check. Spell-check will not catch every error, for example, spell-check will not catch "form" when you really mean "from." Make sure you go back over the script for one final revision to make sure you have the grammar and spelling correct.