(People liked this when I posted it in the Fallout: Equestria group, so I thought, why not copy it into my blog so a bunch more people can see it )
For anyone starting to write, you can save yourself and any potential prereaders a LOT of trouble by putting some effort into self-editing. Editors are also usually a lot more inclined to help out on a story that looks at least halfway decent, and is written by someone who can at least construct correct English sentences in normal conversation situations.
So for all writers out there, beginning or otherwise, I got these pieces of advice. They're basically the same remarks I eventually end up giving to pretty much everyone I preread stuff for, so I might as well put it all together:
Equestria Daily has a very useful document called the "Editor's Omnibus". Especially the "Self-Editing section" contains a treasure of information on correct formatting. I suggest you read it through.
Learn the difference between "its"/"it's", "there"/"they're"/"their", "who's"/"whose", "breath"/"breathe", and, as bonus round, "peek"/"peak".
When using the ellipsis ("..."), ALWAYS put a space behind it. All modern web page renderers and text processors refuse to split on a period, meaning that something like "we got there eventually...but it took us all day." will refuse to split in the middle of the "eventually...but", if that happens to end up at the end of a line; it will be seen as one unsplittable block. The fact the whole thing ends up on the next line looks extremely weird, especially for long words. So, avoid that, simply by putting a space behind every ellipsis you use.
Keep your narration in one consistent tense. A story is either told as it happens, or afterwards. This has certain implications, too. For example, when writing in present tense, you can't foreshadow future events, since a present tense narrator can't know these yet. On the other hand, in past tense, you can't let your narrator be surprised about events happening in the story. You can let him/her tell the reader how surprised s/he was when it happened, of course.
Get a good grasp of basic quoting rules (like the classic "s/he said" construction). This is fully explained in the Editor's Omnibus.
Try to avoid always referring to characters by description of their looks or race. It's okay to use names. When there's absolutely no confusion possible, switch to he/she, but if there are multiple same-gender characters, generally, just use their names. (Mind you, if anyone ever brings up "lavender unicorn syndrome", you have my permission to kick them in the face. Seriously) Note that, of course, you ARE allowed to describe characters. The point is just to not do it all the time, and to never do it if it may confuse the reader. And obviously, it's generally used for characters that haven't been introduced by name yet.
When addressing a person in dialogue, separate the addressing term (usually the name) with commas, both before and after the addressing term (unless either of those happen to be the start or end of the sentence, of course). To make this perfectly clear, compare these two sentences:
"I kill, my Queen," the soldier said to Elisabeth. "That's what soldiers do."
"I kill my Queen," the soldier said to Elisabeth. "That's what soldiers do."
The first is the soldier saying, to his queen, that he kills. The second is the soldier saying he kills his queen. Quite a difference that little comma can make, no? So please, please pay attention to that. If only to avoid accidental regicide
And finally... in any situation, even normal chat or posting in forums... write as if you're publishing. Use correct capitalization on start of sentences and on 'I'. Be your own worst critic when it comes to spelling. Edit your own posts if you see mistakes, and try to avoid them in the first place. If you're unsure if something is correct, don't hesitate, just google it and get it right. This will give you the habit of writing correct English, which will make it much easier to write stories without mistakes.