Previously, I talked about getting feedback for your work. Having your manuscript as close to perfect as possible is an important part of professionalism, whether you're submitting to a publisher or publishing for yourself. That brings up the topic of behaving as a professional, which may be even more important if you're self publishing. Here's some advice from the trenches from someone who's published both independently and traditionally.
1) Only put your best work out there.
If you're working with a publisher, the editor will help you polish your book, and you'll have a copy editor, but you'll improve your odds of being published if your work is just about good to go to begin with. If you're publishing on your own, invest in your career by hiring professionals to edit your work. Don't treat your customers like beta readers. You look amateur when you have high school-level grammar mistakes in a published book or keep issuing new editions as you edit and polish.
2) Develop a thick skin.
This is one reason why I suggest at least trying to go through the traditional process of finding an agent or publisher. Sometimes you need to hear that your book may not be the best ever, and getting rejections helps you thicken your skin. Way too many people have torpedoed their careers by having a raging public hissy fit over a negative review. No matter how brilliant your book is, there will be someone who doesn't like it or doesn't get it. Get over it and move on. Even better, see if you can learn something that you can use to make yourself a better writer.
3) Be careful about engaging with reviewers.
This comes under the category of having a thick skin, but is important enough to elaborate upon. It's okay to thank a reviewer for a positive review. Think about thirty times before responding to a negative review. If there is a major factual error in the review, you might want to gently point that out in private communication, but think about how much that error really matters. If it's a matter of opinion, disengage. You're not going to change a reviewer's mind by throwing a temper tantrum, and you'll just make yourself look bad to potential readers. Believe it or not, sometimes even the F-grade reviews can really sell books.
4) Also be careful about engaging with readers.
There are places where interacting with readers is appropriate, such as forums designed for such interaction or on your own blog or Facebook page. Before barging in on any other forum, take some time to figure out the ground rules of that forum. A lot of reader-oriented sites aren't crazy about authors participating in discussions of their own books because it tends to shut conversation down, even if the conversation is positive. I've noticed this on review blogs. On a site where normally the comments can number in the dozens for both positive and negative reviews, the moment the author chimes in, even if it's just to say thanks for the nice review, all discussion comes to a screeching halt. You don't want that. Buzz happens when people talk about your book. Don't shut down the buzz.
5) And be careful about sucking up (or looking like you're sucking up).
I've seen aspiring or new authors who glom onto bigger names and start name-checking them on Twitter and Facebook or constantly commenting on their blogs. Ditto for blogs/Twitter/Facebook of editors or agents. I'm not saying you shouldn't do this if you really enjoy discussing those authors' books or discussing the topics the editors/agents talk about. But try not to look too much like this is your marketing campaign. I doubt any editor or agent has requested a manuscript due to a brilliant blog comment, and querying via blog/Facebook/Twitter is right out, unless the editor or agent has asked for pitches that way. I've looked at my agent's Facebook page and cringed when I see the number of "could you look at my book?" posts.
6) Behave like a reasonable human being in professional venues.
Even if you're attending a convention or conference as an attendee rather than as a speaker, behave professionally -- without setting yourself up to look like you think you should be the headliner. You may be a fan, but if you want professionals to take you seriously, this isn't the venue to act like a raging fan. At the same time, you're not going to make a good impression if you attempt to throw your weight around when you don't yet have the metaphorical weight to throw. I think just about everyone who's attended a convention has cringed through the scene of the person with one self-published novel who's not even on the program trying to lord it over someone else without realizing that they're attempting to lecture a veteran bestseller about how to be a novelist. Until you're the guest of honor, listen more than you talk.
Just following these few steps will help you avoid some of the worst career-limiting moves or will keep you from going viral as the latest clueless author who gets piled on by the entire Internet.