Proofreading is valuable for several reasons. It acts as a final check against the possibility of your sending out something you really didn't mean to say. (Such as calling your my main character "Delores" in Chapter One and "Rebecca" in Chapter Two.) It also serves to clean up any typos and formatting glitches you may have originally missed before some sharp-eyed editor has a chance to catch them and reject your manuscript as substandard.
So, if you want to have the best opportunity possible of getting your work published, become a better proofreader. It's not really all that difficult, and it will pay off in spades down the road.
Proofreading for content--called, conveniently enough, content proofreading--is probably the most important form of proofing you can do, for obvious reasons. Your work relies upon its content for its success. So, when you're content proofing, look for correct sentence structure, logic, spelling, and punctuation. Also, watch out for inconsistency, which is not so much a problem with shorter works such as stories and articles as it is with book-length material. Inconsistency can be something so simple as using "and" versus "&" on different pages of your manuscript; or it can be the difference in alternating between the use of "ten" and "10" from one paragraph to another.
Proofreading for facts--sometimes called fact checking--is equally important, nearly as much in fiction as in non-fiction. Obviously, if you say in a biography of Abraham Lincoln that the former president was born in 1826 and that's not true, you're going to turn off a lot of people. Similarly, if you claim in a novel that Poughkeepsie is in New Jersey, watch out! Whenever you run across a name, date, place, or other fact, a little red flag should go up behind your eyeballs, and you should make absolutely certain that your facts are correct.
Comparison proofreading offers you the opportunity to compare one document to another, as when you type a direct quote from its originating source. Obviously, you need to be certain that you didn't leave out or add any words or change any formatting from the original document.
Finally, format proofreading is exactly what the phrase implies--proofing for accuracy and consistency in format. If you begin the first ten chapters of your novel with "ONE," "TWO," "THREE," etc., make sure the eleventh chapter isn't is "ELEVEN" and not "Eleven." If you indent your quotes,
"I never met a man I didn't like. I never liked a man I didn't meet. Therefore, I never met and liked a man."
be certain to indent all of them.
Also take a step back and view your manuscript for its overall balance and alignment. Does the copy seem to be weighted toward the left-hand margin? If so, you may need to make a margin adjustment. Are all the paragraphs flush left when they're supposed to be? Are there extra lines of space between some paragraphs? All these things--even though they might sound trivial to you--are important, because they create a strong first impression on an editor.
Remember, when an editor looks at 30 manuscripts a day, the one that's most professionally presented--right down to the overall format and the obvious proofreading effort that went into preparing it--is the one that's going to get noticed.
Here are a few other tips to help make you a better proofreader.
1.) Print out a hard copy of your manuscript and do your proofing off of that. Proofing a computer screen creates too many opportunities for mistakes.
2.) When you find an error or want to make a change, note it on a sticky note, a flag, or right in the manuscript's margins. Don't rely on your memory to go back and make the correction later.
3.) Ask someone else to read your work. A fresh pair of eyes will often find typos that you might overlook.
4.) Read your work out loud. Sometimes, even though the words are all right and the punctuation is perfect, some things simply don't work. Often, hearing the spoken words will reveal any weak spots or awkward phrasing.
5.) When performing a comparison proofread, use a straight edge such as a ruler to move down the lines of the original document. You'll be less likely to miss any text that way.
6.) After making all the corrections, proofread your newly corrected document once more, paying particular attention to the areas you changed...just to make sure you got all the changes right.