We'll start off with the easy ones, shall we?

Check Your Spelling
Practically every text-editing program these days has a spellchecker, as do a whole bunch of sites on the net. Make sure to use one. Even after having run a spellcheck, give your fic a final re-read before you publish it. This is to ensure that you don't have any mistakes the spellchecker wouldn't pick up on, like if you wrote "has" instead of "had". Be mindful of words that sound the same but are spelled differently, like "accept" and "except". Also, make sure to capitalize the right words.

Check Your Grammar
Make sure that all your sentences are in the correct tense, that you've conjugated all your verbs correctly, and that it's always clear which person or object you're referring to. These things can be tricky, especially if English isn't your first language. If you're not absolutely sure that your grammar is at least 95% correct I recommend getting a beta reader to look at your work before you post it anywhere.

Make Sure Your Sentences Are The Right Length
Long sentences are fine. Short sentences are fine. Just don't use too many long or short sentences in a row. If you use too many short sentences your story will. Seem. Chopped. Up. Into. Little Bits. Which. Are hard to read. If your sentences are too long the reader will get bored halfway through. If you use too many long sentences in a row you might sound pretentious. Try to srike a balance between the two.

Observe these three things and even the worst story will become readable.

Now, on to the more complicated things...

Check Your Vocabulary
First and foremost: Don't use words if you're not sure of their meaning.
If you've understood the word wrong, sure, it gives the reader a laugh, but it'll make you seem careless or incompetent.
That said, do try to have as much variety in your vocabulary as possible. Draco doesn't just smirk, he sniggers, chortles, sneers, laughs, guffaws, roars with laughter, etc.
If you're writing dialogue, make sure the vocabulary is appropriate for the person using it. For example, Snape wouldn't use the word "kids". He'd say "children".
Also, for all you non-Brits out there:
We British people absolutely hate reading Harry Potter fanfics where the language is all American-sounding. You know, where people say "mom" instead of "mum", "vacation" instead of "holiday" and "candy" instead of "sweets" and "chocolate". So, if you're not British (it's not just Americans who do this) and you want your fanfic to seem more authentically British, ask a friendly Brit to look at it for you (you can find many of these friendly Brits in the Britpicking section at Fictionalley Park).

Check Your Characterization
Bad characterization is a disease suffered by far too many pieces of fanfiction. Only very few stories that feature someone who is out of character give a reasonable explanation for that character's unusual behaviour.
You should constantly be asking yourself if the character you're writing about would really act the way you are writing about him/her acting. Would Draco really react to something in that way? Would Hermione really say something like that? Would she really wear something like that?
Something that helps with dialogue is to imagine the actors from the movies saying the lines you've written for their characters, mentally aging them if required, of course. Can't imagine it? Then change it, it's out of character.
Above all, remember that the character you're writing about is not you. Just because you would say yes if Draco asked you out doesn't mean Hermione would. Just because you think Draco would look good in Muggle clothes doesn't mean he would actually wear them.

Check Your Plot And Continuity
Obviously you get bonus points for original plots, but the old classics can work well too. The most important thing is just that your plot makes sense. It can seem completely illogical to the reader at the beginning, of course, if that's the way you want to write it, but at the end everything should be clear. The reason why Draco and Hermione fell in love, for example, or how exactly Draco escaped Voldemort's evil clutches. This is where continuity plays a big part. If Draco escapes Voldemort by using the Avada Kedavra curse, there should be some mention of him being able to perform this curse earlier in the story. It is, after all, very difficult. The opposite is true as well. You shouldn't describe something in too much detail that will never be mentioned again, the potion Snape is making them study in a particular lesson, for example. Another important thing is to make sure your timeframe is right. If Draco is 16 at the beginning of your story and Hermione gets pregnant five months later, he can't still be 16 when the baby is born.
To help avoid problems like these, it helps to plan out your entire fic on paper or on your computer before you start. You don't have to put exact dates on every event, but writing down the important things that happen on each day, week or month (depending how big your timeframe is) is really useful.

Check Your Perspective
You should know from the beginning if your story is going to be in the first person or the third person and whose point of view it's going to be in. Is your story entirely in one person's point of view, will you be changing points of view, will it be from everyone's point of view at the same time (an omniscient point of view)? If your story is from Hermione's point of view, you shouldn't suddenly put "Draco felt very impatient as he sat there on the steps waiting for her". How would she know? She wasn't there! Only write things that your character knows, suspects or feels. If you're changing points of view, make that change clear by putting the new point of view in a new paragraph. However, don't write things like ~*~*~Draco's PoV~*~*~. Not ever. The reader should be able to tell from the text whose point of view you're in. Besides, things like that make the author seem lazy and unprofessional.

Mary Sues Are Bad
What's a Mary Sue, you ask? Well, I think the Mary Sue Litmus Test explains it pretty well. If you have any original characters (characters you invented yourself) who play more than a tiny part in your story I strongly advise you to take the Mary Sue Litmus Test. While there are some authors who manage to make Mary Sues work for them, the appearance of a Mary Sue in a fic usually spells instant badness. Avoid them, they're annoying and make many people stop reading. In fact, avoid any original characters as much as you can. You'll find you can tell most (though not all) stories just as well with J.K. Rowling's already established characters. Think carefully before creating an original character about whether this character is really necessary to the plot.
 

And finally, just my personal pet peeve...

Don't Mix Script And Story Formats. Ever.
Example: Draco sat down. Draco: I'm sorry Hermione.
First off, if you're writing a script you use the present tense for the stage directions (the bits that aren't people talking). No exceptions, at least not for me. If you're not writing a script you do not just write the name of the person in front of what they're saying. You put something like "Draco said: 'I'm sorry Hermione.'"
However, you can write a "normal" story in either the past or the present tense.
 

Helpful? Not at all? Anything you'd like added?
E-mail me at greatdhfics@yahoo.com