1) Write a play with nothing but unpleasant characters. This is an age of anti-heroes, after all. Make sure there is no one onstage that an audience could possibly like or want to spend time with. If they wanted to be comfortable or happy, they should have stayed home.
2) Choose a topic that you think is marketable but you don't really care much about. After all, a playwright should be able to crank out something mildly entertaining without a strong point of view. Something like an episode of your favorite sitcom should go over well, don't you think?
3) Write a play that requires a realistic set change every three or four minutes. Or that has at least two or three insurmountable props, like a driveable car that goes on and off stage. Or lots of cool special effects. If Miss Saigon can have a helicopter that hovers and lands onstage, why can't you?
4) Don't include a cast list at the front with the names and number of characters; after all, you wouldn't want the theatre to be intimidated by the cast size right away. Let them discover the vast army of characters by reading the play. Make sure there are plenty of characters that have only one or two lines. After all, actors need work.
5) Don't number the pages, either. Let the theatre guess how long it'll take by hefting it. Anyway, 160 pages isn't all that long, is it? Especially when the play is in 5 acts and 23 scenes.
6) Leave the pages loose, or stuck together with a paper clip that easily falls off. (This is especially effective when you're been diligent about rule 5 above.)
7) While you're at it, invent your own play format; the one from Samuel French or Dramatists Sourcebook is sure to be too confining. Be creative with your spelling and grammar, too. All of this will show an irrepressible original mind at work.
8) Open your play with several pages of stage directions, in long impenetrable blocks. Describe the sets and furnishings with such numbing detail that the set designer will know exactly where to buy the priceless antiques you need for scrupulous authenticity.
9) Make sure that the first 10 or 15 pages is nothing but exposition or trivia by minor characters. Most audiences don't settle in or stop rustling their programs until 15 minutes into the play. Don't give them anything meaty until their bottoms have conformed to their chairs.
10) Put in lots of stage directions for every speech, indicating exactly how you think an actor should say the line. Example:
JANE: (coyly) No.
JOHN: (very angry, but holding it in) Why not?
JANE: (flirting more hesitantly now) Because.
JOHN: (swept away with passion) All right, then.
11) Be sure to include at least one 3-page monologue, per character, in every scene. And do keep all the characters onstage whether they have anything to do or say in the scene or not; after all, the actors need to practice concentrating on listening intently for a half hour without dialogue or stage business.
12) Make the dialogue as generic as possible. You might, for instance, write an absurdist play where all the characters are named MAN and WOMAN 1, 2, 3, etc., and they all spout general philosophical abstractions until it's hard to tell their characters and dialogue apart. That way it will be intellectually deep and universal and everyone will be able to identify with it.
13) Alternatively, base your play on your own life, particularly your frustrations, and how no one understands and appreciates you. Don't change anything; people need to experience unvarnished reality.
14) Print your script on a dot-matrix printer in which you haven't changed the ribbon in years. Use a "creative" font, like all italics or cursive. Then photocopy it on the lightest possible setting, to conserve toner. Even better, write your script in longhand. Anything to make it to stand out from all the others.
15) Send your script to every theatre you ever heard of. Don't bother finding out what kinds of plays they usually do; they're bound to love your masterpiece, no matter what. After all, everyone, from a radical experimental company to a Shakespeare festival to a community theatre that only does musicals, NEEDS to experience your gripping 53-character historical play about Civil War amputees.
16) Diligently follow up by calling the theatre every week or so until you're sure your script has been received and read. That way the staff will be sure to remember your name.
17) Leave your address and phone number off your script, and don't include a SASE (self-addressed stamped envelope, manuscript-sized) for return of your script either. That way, you'll never have to face rejection -- because the theatre won't be able to find you.