Or: more rubbish from me.
Stuff not to do unless you're in love with the idea of being rejected and want to marry it:
1. Go on your blog and do a big rant about how everyone in the publishing industry is a massive idiot. Then name names, and describe why they're an idiot. The description will usually include things like "because they rejected my book" and "because they rejected my book" and then it'll finish with "because they rejected my book". It does not. Make you. Look good.
Not only that, but doing stuff like the above will only cause you drama. And drama may seem cool when you're fanficcing on livejournal and everyone has a big fight because someone said summat idiotic like "gais r baddies", but it is not cool when your career depends on something. It will make you ill. Stay calm, don't get involved in internet fights, don't respond horribly to reviewers, try to keep your crying about stuff to a minimum (that last one I fail at horribly, but I do my best. Twitter is forever but it's less forever than a ten page blog post).
2. Start your novel with a massive info dump. Agents and editors will forgive a lot of things, but I rarely see them forgiving this. And why should they? If they've got book A starting off "The world of Tengongtwon is a big pizza floating in space and on it live all these people called the Pizzonians and Mary is one of them. She lives in a big house by a lake and she has blonde hair and big boobs and she's 5"10..." and book B that starts "Mary looked up at the sky and wondered if there really was a Pizza God, the way everyone said" they're not going to pick book A. They're just not. Book A leaves nowhere for them to go. Nothing for them to find out. Also, it sounds like nonsense - where as book B at least sounds like it might be going somewhere interesting.
And don't try to hide your info dump, either. "Mary flicked her blonde hair and adjusted her top over her double D boobs and then noticed how tall she was in the window next to her" is just as bad. Pace your information. Focus on the action.
3. Query letter and formatting mistakes. If you write a query letter that's ten pages long, and has the synopsis on page nine, and doesn't include any pertinent information about you (publishing credits, area of expertise related to the novel), and then your first three chapters are in font size 18 with pink Gothic lettering and eight million typos, you know what's going to happen? Your masterpiece will not even be read. That's a fact. That's not me exaggerating. I know this has actually happened a million times over - just read Query Shark.
Agents and editors don't have time to read something from someone who hasn't done the barest minimum of research into querying, MS production and publishing. Think of it this way: if you owned a business and someone filled out an application for a job that ranted for four hours about how much they loved burgers when what you made was pizza, would you really want to hire them? Could you even be bothered to give them an interview? I don't think so.
4. Send a sci-fi book to an agent/publisher who only deals with historical novels. Same thing as the burger/pizza analogy. It's just not going to happen. They're not going to change their minds for you. All you'll end up with is another rejection that whispers to you in the night that it wasn't because you sent a sci-fi to a historical publisher. It was because you're shit, you'll always be shit, no-one likes you and you should go die in a fire.
5. Write a book over 120k for your first submission to publishers. Stephen King gets to publish novels over 120k. Because he's a living legend. It's not even taking a chance, to take a chance on him. Publishers are taking a big chance on any first time author, and that chance gets a million times great every K you go over 120.
Though I should add, here, that there are books that have gone over 120k and been published by first time authors. There are. Hell, I'm sure there are books and authors who've broken all these rules and still gotten a deal. But the thing is: why be the exception to the rule? Do you know how hard it is to be the exception to the rule? It's the exception because it's rare and tiny. Why aim for rare and tiny?? If you've written a book and honed it and loved it and worked on it for a thousand years, then someone says to you right, you now get to make your own bullseye and once you've hit it, you win, you don't make a bullseye a milimetre across eight thousand miles away.
You make it as massive and close to you as you can. You make it two inches from your face and fifty feet across. Don't make things hard for yourself. In this case, you're not being cool by trying to be the exception to the rule. The rules are there to help you be awesome. Don't fail to be awesome.