Professionalism is a variable defined by the profession.
If I'm a professional wrestler, for instance, professionalism includes never going out in public without the "PERSONA."
If I'm a professional juggler it means I generally don't show up for a performance in a business suit.
If I'm a professional golfer I don't do a happy dance after making a birdie.
Professionalism is a certain set of expectations as defined by the field of study or business.
If you want to be taken seriously in that profession you follow that profession's rules of professionalism. And these rules apply to much more than just professionals. It applies also to all those who wish to be encompassed by that field. For example, bank tellers don't wear ripped jeans and KISS t-shirts to work. Construction workers don't wear Speedos on the job site--as much as, perhaps, we might want them to.
In the writing business there are the pros and the wannabes. And both should present themselves professionally when seeking publication.
As I'm sure you know, writers tend to be somewhat eccentric. That's fine. There is no strict dress standard for writers. If you show up to a contract negotiation table in a tie-dyed sundress in January, nobody's likely to bat an eye. You ARE an artist, after all. They're used to artists. That said, I'd consider carefully your purposes if you decide to push that envelope TOO far. You still want to present yourself as relatively sane, reliable, productive. It IS still a business, after all. And those involved have business interests to protect, such as, Is This Writer Sane Enough to Actually Finish a Manuscript?
In reality, most writers aren't actually seen by the production end of industry professionals anyway. You can be an 800 lb professional couch potato and still write novels. BUT, a successful writer WILL be seen and judged by his/her public. So there's also a level of professional conduct and personal presentation that goes along with that. What does your audience expect of you? That may be determined by what you write.
Personally, I've enjoyed seeing the authors who let a bit of their eccentricity come out: the high-fantasy writer who sports a chest-length beard and a gold hoop earring while wearing a business suit, for example; the sci-fi writer who wears Star Trek t-shirts; the gardening book writer who comes in a huge floppy sunhat, huarachi sandalls, and dirt under her fingernails. But what if that last author wrote high-tech crime drama? I'm not sure I'd believe her capable of doing so. The persona she presents to me doesn't say "nerdy technical novels." Author number one might. Author two possibly.
So there's something to be said for presenting yourself in accordance with potential expectations. But is that professionalism? I think so. If you want to make an impact in the business you'll tweak some things about yourself to make yourself saleable, while still being true to who and what you are.
But what I really wanted to write about is selling your story.
Stories sell themselves.
Sure. You can wish.
Authors first sell themselves, and THEN the story sells itself.
The first part--the author selling himself--is easy-peasy-rice-and-cheesy.
It's all about professionalism and presenting a manuscript professionally.
This is done in two ways:
1. The Query/Cover Letter
When submitting a story, your cover letter is a first impression. Make a good one.
Keep it short, sweet, and to the point. The cover letter should NEVER be longer than one page, complete, WITH addressing and signature. It should contain the title of your story and any publication credits you have. That's all. It's purpose is ONLY to be informative. Most editors don't carefully read cover letters anyway. They usually scan them looking for clues that you know what you're doing, that you know how to present yourself professionally. Begin your cover letter with respect. "Dear Mr. Editor, I respectfully submit my story (or novel, or article), "Title," for your consideration."
A Query letter is something quite different. The purpose of it it is to sell INTEREST in your story. In this case, the letter should ALSO be short, sweet, and to the point, similar to the Cover letter, but you may include ONE extra paragraph (generally no longer than 25 words long--YES, you read that right) summarizing your plot. You will want to BEGIN your letter with this paragraph to immediately draw the editor in.
*Use the editor's name in the letter's address if possible: "Dear Mr. Johnson" or "Dear Ms. Baker."
*Include professional background relating to your hopeful publication. Include publishing credits (especially those relevant to your market, ie. literary publications when submitting to literary markets--and on that note, most literary markets will be completely unimpressed if you include sci-fi/fantasy/romance publication credits); degrees relating directly to writing or the submission's content (ie. a Master's Degree in history if you're writing a nonfiction historical); workshops you have attended that the market may be impressed by; other experiences--either professional or volunteer (Do you run a writer's workshop for elementary school children? Do you work as an writer and/or editor for a magazine or newspaper?)
*Use proper letter format, with your full name, address, phone number, email address, story/book title and word count in the top left-hand corner of the page. Most word processing programs come equipped with business letter templates. This IS a business letter. You are hoping the publisher will want to do business with you.
*For that extra special touch, close your letter by thanking the editor for his/her time.
*DO NOT print ANYTHING on ANYTHING but bright white paper.
*DO NOT print ANYTHING in ANYTHING other than standard manuscript fonts--like Times New Roman, Courier, or Courier New.
*DO NOT include pictures or symbols or designs. Just a plain white piece of paper with plain black lettering on it. This is especially pertinent to electronic submissions. A dolled-up electronic submission can take 100 times the memory space that a plain submission can. Even in this day and age of 500 gig computer hard drives, memory can get used up fast if everyone submits that way.
Below are some links to some articles and/or sample query letters:
Sample Cover/Query Letter 1
Sample Cover/Query Letter 2--Gives an example of a bad letter and a good one.
Sample Cover/Query Letter 3--from Sullivan Maxx Literary Agency.
2. Manuscript Preparation
Three words: Standard Manuscript Format.
Standard Manuscript Format--I disagree with the rigidity on font from this particular source. Courier is going the way of the dinosaur and not all word processors support it anymore. Hence, Courier New. Times New Roman is also an industry standard font--the one that as a writer and editor I prefer, actually. At any rate, this site addresses both short story and novel format.
Manuscript Format--a guide for hard-copy and electronic submissions. Yes, there are differences.
One more word of advice on Manuscript Format: It all depends on your market. If they specify formatting, follow their specifications to the letter. If they don't specify formatting, I guarantee they won't be disappointed to see you making yourself LOOK like a pro by using Standard Manuscript Format.