It isn't necessary to file a copyright with the Library of Congress.  Your script is automatically protected under common law.  However, it's a good idea to register it, either with an online service, such as the National Creative Registry (protectrite.com), or with the Writers Guild of America.  This being said, the Industry tends to view registration and copyright notices as the marks of a paranoid amateur.  You would be wise to leave them off your script.

Use a plain cover.  White or pastel card stock, not leatherette.  Avoid using screw posts, steel fasteners, or plastic-comb binding.  Bind your script with sturdy, brass prong fasteners, such as those made by ACCO®.  The ones Staples sells are too flimsy.  Readers hate it when a script falls apart in their hands.  You can order professional-quality script supplies from WritersStore.com or the ScriptBuddy Store.

Kill the graphics.  No pictures on the cover or within the script.  This is a dead give-away that the writer is an amateur.

Although scripts are printed on three-hole-punched paper, there’s an unwritten rule that speculative scripts are bound with two fasteners, not three.  Why this tends to be common practice is unclear.  Perhaps it’s because submissions often get copied by the studio's story department, and it’s easier (and cheaper) if there are only two brads.  It’s an indication of how petty some readers can be that they judge your professionalism by the number of brads you use.  However, to avoid this pitfall it’s a good idea to use only two brass fasteners to bind your script.

Use a basic fly page with the script’s title, the writer’s name, and contact information.  No more, no less.  The title should appear on line 25, centered, in quotes, and in ALL CAPS.  There should be four blank lines between it and “Written by” (also centered), and one blank line above the writer’s name, which should be centered on line 32.  The contact information should appear at the left margin, its last line being an inch from the bottom of the page (i.e. line 60).

The draft date is not needed on a speculative script (as opposed to a shooting script), and may be left off the fly page.

Use a fixed-pitch, Courier typeface.  Movie scripts still look as if they’ve been written on an old, Smith-Corona typewriter.  Some readers actually dread proportional spacing, as it allows writers to cram more text onto a page.  A proportional spaced typeface may appear more polished, but the standard is 12-point Bitstream® Courier 10 Pitch (not Courier New, which is too spindly).  Hewlett Packard distributes a free “Dark Courier” True Type font based on Bitstream® Courier BT.  You can download it here.

Page numbers go at the top, aligned with the right margin, and followed by a period.  There is no need to preface the number with the word “Page.”  The page numbers should be in the same typeface (see previous point) as the text.

The page count begins with the first page of the script, not including the fly page.  The page number should appear on the fourth line down from the top edge.  No page number should appear on the first page.

There’s no need to put the title, draft information, and date in the header. It’s only required for “A” and “B” pages when a film is going into production. For spec scripts, the page header should have only the page number.

It’s customary to place the title at the top of the first page, centered, underscored, and in ALL CAPS.

Standard practice is to begin the script with the words “FADE IN:”  There should be only one blank line between this and the heading of the first scene.

The first line of text should appear on the seventh line from the top of each page.  The bottom margin varies, according to the rules for where it’s permissible to break a page, but the target is between half an inch and an inch.

Two spaces should follow the punctuation at the end of each sentence.  (Don't confuse this with double-spacing the lines, which is done only in three-camera television shows.)  Keeping sentences separated by two spaces, not just one, makes the script easier to read.  Two spaces also follow a colon.

Underscore for emphasis instead of using bold or italics.

Print your script on only one side of the page.  Double-sided printing may save paper and make your script appear slimmer, but readers tend to find it awkward and annoying.  It takes twice as long to turn a page, which may result in the perception your script reads slow.

Try to keep it under 120 pages, but no shorter than 100 pages.  Longer screenplays used to be more acceptable.  (The final draft of Chinatown, for example, was 145 pages.)  However, the trend is toward shorter, punchier scripts.  The rule is a page per minute.  Comedies tend to be shorter than dramas.

Don't cheat by narrowing your margins in an effort to shorten the page count.  The standard width of a dialogue element, for example, is 33 characters.  Narrower margins make it more difficult to estimate the running time.  Even worse, a wide swath of dialogue forces the reader to spend more time on each page.  This may also convey the impression your script reads slow.

Use the standard pica line spacing of six lines to the inch.  While word-processing software may permit you to compress the lines to fit more text on a page, closely spaced lines are harder to read.  What’s more, tight spacing will throw off the estimated running time.

The top “CONTINUED:” and bottom “(CONTINUED)” should be omitted.  They are needed only in shooting scripts.  As is the case with scene numbers, these notations aid the production staff in scheduling the shoot.  In speculative screenplays intended for submission, top and bottom “CONTINUED’s” only clutter up the page.

End the script with the transitional instruction “FADE OUT.” (including the period).  Insert three blank lines, and then write “THE END” centered, in ALL CAPS, and underscored (but without the quotes).

Run a spell-check.  All it takes is a few mouse clicks, but it's surprising how few writers do this.  Simple spelling errors may be interpreted as carelessness on the part of the writer.  And if the writer doesn't care, why should the reader?

Make your job easier by using software dedicated to scriptwriting.  Final Draft® and Movie Magic® Screenwriter™ have become standards in the Industry.  The Cinergy Script Editor, a free download from Mindstar Productions, integrates with their line of production software.  These programs automate the formatting and maintain consistency.  They also remember your choices and offer keyboard shortcuts for quickly inserting such elements as scene headings and character cues.  No matter what software you use, though, it won't handle all the formatting.  Some format rules go beyond margins and pagination.

If you're on a Mac, you can also take advantage of the submission tools in Montage:

You can use the Web-based ScriptBuddy.  It requires nothing to install, works on any Internet-enabled computer, and makes collaboration easy and convenient.  ScriptBuddy automatically backs up your script, and stores it securely online.  The free trial lets you write one script.

Celtx is one of the more exciting newcomers.  Not just a screenplay formatting application, it's an entire creative platform that integrates pre-visualization tools, production scheduling, and the Internet.  It has everything you need to take your story from concept to production.  And it's completely free.