This week's completely obvious, often-repeated shibboleth: "The best writers are good editors." In other words, good writers know how to edit their own work.
This self-editing process is bifurcated, however. First, a novelist has to go back and look at the manuscript - and revise, and revise, and revise, before ever submitting. Second, a novelist has to be able to take an agent's and an editor's suggestions and do something with them.
I'm not sure that these skills can be taught, or easily summarized in a blog post. But there's one thing that I can emphasize, and it'll apply to almost everyone who submits their first manuscript to an agent or publisher:
CUT YOUR MANUSCRIPT BY AT LEAST 10,000 WORDS. OR MORE. MAKE IT SHORTER. THEN SHORTEN IT AGAIN.
The vast majority of manuscripts are too long. This isn't to say that they're necessarily "self-indulgent," though some of them are. Rather, length comes from being too focused on what you're doing as a writer and not concerned enough with the reader's experience. You may think, "Oh, I'm setting this up - I'm setting that up - I'm giving necessary background - I'm building character." If it causes the editor's eyes to glaze over, it needs to go. Not just in the early pages of the manuscript, but throughout the book.
A lot of writers are loath to give up a scene in which the description is particularly affecting, or characters engage in witty conversation, or the author philosophizes about a contemporary issue. But these passages will be ripe for the red pencil if it ever gets to the "I'm serious about possibly publishing this book" stage of the game. Why not increase your chances and get rid of all this extraneous "stuff" right from the beginning?
I know this industry beats up on people. Just when a writer's skin has been thickened a layer or two, out comes a cat o' nine tails to strip another few layers off. First, the pain of working the writing into your schedule. Then, the travails of a critique group in which people may or may not have any idea of what they're talking about. Then one-sentence dismissals from agents, or much-too-honest feedback from those agents who might be interested - IF you can fix things to their satisfaction. Then the pain of rejections when the agent submits - and THEN the editorial process at the publishing house, where the editor (if s/he's the type to pick up a pencil) starts excising favorite paragraphs, pages, or even chapters. And then the torture of sales and marketing... and the worry about whether you'll get a contract for your next book. It's like getting to the seventh circle of hell and then having to start all over again at the first circle.
So - why not avoid (or at least greatly lessen) all those slashing red marks by being proactive about it? You'll save yourself some emotional pain later in the process - and you're more likely to get people interested in your manuscript to begin with.