Should I send a brochure or set of comments from workshops on my script, or a tone-setting document, or a videotape of the production of it, or newspaper clippings announcing awards the script has won at community theatre level, or mention of professional notice parts of it have received? 

Or should I just count on the script to sell itself?

A: Years ago, a junior executive said to his boss: 

"We got this thing called Coke. Should I advertise it, or should we just figure people will know it tastes great?" 

The boss said, "If you have to ask, you're FIRED." 

No script sells itself. YOU sell it. Nobody would know who Samuel Beckett was if his dutiful wife hadn't taken copies of his scripts around and INSISTED that producers and directors read them. (True story.) Unless you've got Beckett's wife, you can't afford to be Beckett. 

ABSOLUTELY notify literary departments of readings, wonderful reviews, feedback, word of mouth, God raining manna on you, everything. Don't deluge literary departments with things they don't want -- videotapes, scripts, blood samples -- but provide them with simple, eyecatching, easy-to-read praise. After 12 years of getting my plays produced, I have a lot of reviews I can send out with scripts. Two years ago it occurred to me that including all these good reviews was actually to my detriment -- nobody was going to read them all. And I didn't want to leave any out because, well, they are good, and they're for a variety of plays and from a variety of sources. So I took a clue from a performance-artist friend (who started doing this maneuver for granting agencies): I developed a blurb-sheet, just like you see on the back of acclaimed paperback novels. It's still three pages long, but it's easy to read, it's got headlines, it's got sources listed it, and it whets their whistles for reading my play. And, I enclose a full review or two so they see I'm not lying. 

Study your Playwright's Companion and Dramatists Sourcebook and Dramatists Guild Resource Directory and Market Insight... for Playwrights as if those publications hold the secrets to the universe. Then send to the relevant theatres exactly what they ask for. If you've got more (a videotape that you think is actually good), offer to send it if THEY request it -- and encourage them to contact you by email. It's simpler than snailmail and it's a good step toward a personal relationship. 
Back to the questions

Is format, neatness important in selling a script?

Neatness isn't important -- it's essential. And proper playscript format is right behind it. Literary departments have hundreds or thousands of scripts to get through at any time; don't give them an obvious incentive (i.e., eyestrain) to send yours back right away. 
Back to the questions

I've worked in multiple literary departments where the cover letter and resume accompanying submitted scripts, if any, are promptly filed and/or discarded before the script is farmed out for reading. So I thought I'd ask: do YOU find that the quality of a cover letter or the impressiveness of a resume/bio influences your reading of a script or query? If so... what do you personally find appealing, helpful, or otherwise in a cover letter?

Personally, I have no interest in cover letters. I don't even read them. My overwhelming interest is in the script; a secondary interest is whether or not I've heard of the playwright before. (I comb through the list of productions in the Dramatists Guild Quarterly with almost religious devotion.) I'm interested in industrious playwrights who are committed to their craft, and to me that usually means they're getting productions, readings, workshops, etc. around the country (and world). However -- that certainly hasn't stopped me from producing dozens of first-time playwrights in the past five years. It's just a (minor?) factor. I should also add that the literary director at my theatre feels strongly about the worth of cover letters and reads them carefully to help him get a handle on playwrights he doesn't already know. We agree completely, though, that for both of us the important thing is the SCRIPT and not the cover letter or the resume. 
Back to the questions

I've been writing plays for two and a half years and have had six productions so far. In terms of more efficiently marketing my work, should I consider looking for an agent, and if so, how?

I've had an agent for I guess five years now (six?) and she hasn't gotten me a production yet. I know a playwright who's been done on Broadway who says that in 30 years of playwriting an agent has NEVER gotten him a production. Believe me, agents by and large don't get you productions. You and your work get you productions; an agent negotiates deals. You will work far harder on getting your work done than any agent ever will, and if you won't do so, you're doing yourself a disservice. In fact, right now you should stop reading this email and pick up your Playwright's Companion 1997 again. 

As for whether or not it's time for you to pursue an agent, that depends. How much "buzz" do you feel about yourself lately? If your honest answer is "none," I'd advise against. Getting an agent means writing a letter to any of 30 or so agencies and familiarizing them with your work. "Dear Ms. blah blah, I am currently seeking representation. I've had xxx plays done over the years. My most recent, "Snort," won the 1997 Snodfarb Smelgrass Award and is scheduled for production at the Off-Beat in New York City this February. Would you be interested in seeing a copy of the script? etc." (And then always enclose a SASE.) The potential problem here is this: If you've contacted them once and you are NOT ready for an agent, they'll sniff you out so fast you'll squash your chances for next time. My advice: Wait until something BIG is happening with your career. Your new play off-off-Broadway is getting lots of attention. Your Equity 99-Seat Plan show in L.A. is drawing lots of film and TV industry people (and believe me, that'll get an agent's attention). You've won an award. The New York Times or L.A. Times or Times of London or L.A. Weekly or Village Voice or American Theatre has done a feature about you. You get the idea. 
Back to the questions

What percentage of one's time should a playwright who lacks representation spend on marketing?

You should seriously apply yourself to marketing your work at least once a week -- if you've got enough plays to make that time worthwhile. At the moment I've got 46 plays out to theatres in the U.S. and London -- all of them requested by the theatres. That means that either they heard of me or the play and wrote to me; they emailed me; or they responded to a query letter and synopsis. (Most of them were from the first two methods.) I work hard at getting my plays produced for a very simple reason: If I don't have a production (or reading or workshop or fellowship or something) on the horizon at all times, I get seriously depressed. I re-evaluate the time I've spent writing these stupid plays. I start to wonder if I wouldn't just drive off a cliff if not for the fact that it would forever damage my son's future. I look at my production resume and suddenly 36 productions look like, well... nothing. (I'm not kidding.) So just for my own mental health and the well-being of my poor wife who has to live with me, I apply myself to marketing my work. I have a play running here in L.A. now, a different play opening in northern California in September, meetings in New York in August and September, and a third play opening in New York in late January. So until that last one ends, my mental health is relatively assured. Now I need to line up something for after that. 
Back to the questions

Since we all realize that time spent marketing is time spent not writing--can you suggest strategies for efficient marketing/promotion?

First, I wish that more playwrights would realize that other playwrights aren't the competition. I know that sounds strange, but we aren't. No two plays and no two playwrights are alike. Meet other playwrights and trade resources. I've gotten a number of productions and readings out of this. 

Second, be friendly with everyone you meet in the theatre. Just because someone else is a jerk doesn't mean you have to be. And sending a little peace, love and harmony out into the ether often comes back to you later -- in the form of productions and readings. 

Third, study those theatre resources directories, be honest about the relative strengths and weaknesses of your play in relation to those particular theatres, and submit accordingly. Don't send complete full-length scripts. Send a synopsis and the best damn 10 pages therein (not necessarily the first 10). Think advertising. You are trying to hook them into requesting the full script -- so cut down on the verbiage in your submission package. Simple cover letter, simple synopsis (NOT a step-by-step breakdown of what happens; trust me: they're looking for the THEME of the play, and for it to sound dynamic), and a postcard. Suggest to them that they can email you. (My most recent notification of an offer of production was via email.) 

Fourth, find a theatre and get involved. Every playwright needs a base of operations: support from others who like his or her work, and access to actors and directors. If one isn't immediately close to you, be prepared to drive once in a while. Having a base of operations has made all the difference in my own career.