FF.net has a lot of work on it; some of it good, most of it bad. Here are a few tips to make your story better:

1. Summary

When people look through a list of stories, the most important deciding factor is the summary. Don't write 'I suck at summaries', because it indicates that you're probably not very good at writing at all. Don't put the summary inside the story: remember that on the internet, acceptance and rejection are merely a click away, and potential readers don't want to have to open your story just to realise they're not interested anyway.

2. Grammar

I shan't include a full grammar here, because I can't be bothered, it'll take too much space, and you're probably not interested. However, I will say this: read everything aloud (or at least mumble it) before posting. If at any point you find yourself taking a breath, you need a punctuation mark there. Here is a brief summary of punctuation:

Full Stop or Period: This is used at the end of each sentence, and historically after each letter in an acronym, although this latter practice is now optional. It may also be used after abbreviations (for example, Mr.). For these last two uses, be consistent. If you are unsure as to what defines a sentence, these three definitions are useful: a phrase is a series of words with meaning, for example William's yellow book; a clause is a phrase containing a verb, for example after I bought the cake; and a sentence is one or more clauses that express a complete idea, for example, after I bought the cake, I threw William's yellow book at the one-legged rhinoceros. (A word that is commonly mistaken to not be a verb is the verb to be, and all its conjugations, like am, are, was, and were.)

Comma: This is used to join clauses or phrases in a sentence, for example, William's yellow book, which I bought for him along with a cake, collided with a one-legged rhonoceros's head. It can also be used to separate list adjectives, for example, the yellow, spotted, smelly book.

Apostrophe: This is the most commonly misused and abused punctuation mark. It has two uses: in contractions, such as can't, the contracted form of can not; and to indicate possession, technically known as the genitive case. For example, the word Dursley's can only refer to something owned by a person known as Dursley. To refer to the family itself, use Dursleys. To refer to something that belongs to the whole family, or, as an extension of this, to form a colloquial metonym for their house, use Dursleys'. General rule: form the plural, then add the apostrophe on the end.

Colon: This is used to indicate a clarifying or amplifying clause or phrase. If the succeeding phrase or series thereof is of significant length, the first word may be capitalised. The colon is used to indicate a pause more significant than a comma, but less significant than a full stop. Subjects, objects and verbs cannot be assumed across a colon.

Semicolon: This is used in lists where each item is a clause rather than a single word, for example, He held in his hand a compass for navigation; two maps, one large scale and one small scale; a large sextant for finding longitude; and a diesel submarine. They can also be used to devide clauses that are both clarifying and successive.The semicolon is used to indicate a pause more significant than a comma, but less significant than a colon, and subjects, objects and verbs cannot be assumed across them.

Quotation Marks: There are two types of quotation mark: single and double. Contrary to popular belief (yet another proof that the mob is almost invariably wrong), the single quotation marks are to be used in almost all circumstances, for example, 'Never!' he bellowed Double quotation marks are only used when the speaker is something else, for example, 'Listen to this: "The forty thousand pounds due to the heir of Spon are still unclaimed," she whispered. Quotation marks should be used for all forms of speech as well as thought. They should be concluded with another punctuation mark, usually a comma, which is used in place of a full stop to prevent unintended premature ending the superordinate sentence.

Exclamation Marks and Question Marks: Exclamation marks indicate shouting or agitation, and question marks indicate a question. Do not use more than one in any instance. Do not use them together; the question mark has precedence.

Make sure that the first word of every sentence is capitalised, and that proper nouns (the names of specific things like people and places) are also.

All of these rules apply to summaries as well.

3. Style

Tense: While there are six tenses in English, we only use them in three categories: Present, Past and Future. Stick to one tense. Most stories are written in the past tense. If you irrationally or accidentally change tense, your readers shall get confused, and confused readers give up.

Person: In grammar, there are three persons. First person has the persona as the narrator: I got up to brush my teeth. Second person (which is very rare except in dialogue) has an accusatory tone and is only used to draw the reader into the crux of the scene: You got up to brush your teeth. Third person is the most common used in fiction, and has a detached manner: Sarah got up to brush her teeth.

Paragraphing: While a sentence expresses a complete idea, a paragraph is a group of sentences that express an all-encompassing idea. When writing for the internet, make sure there is a blank line between each paragraph; this makes your story easier to read. In dialogue, each time a different person starts to speak, a new paragraph is needed.

4. Miscellaneous

Don't end a chapter where you get bored of writing. Every chapter ought to contain something integral to the story, whether it is a foreshadowing, character development or actual plot advancement. Make sure your chapters a decent length, otherwise your readers will get bored of waiting for something that gives them nothing anyway. A cliffhanger is a good technique, but don't overuse it.

Dialogue is people talking. Read it back, and if it's worded in a way that you would never say, it's wrong. Unless, you're a Latin student who aspires to be the next Cicero (like me), you don't speak in essays. I only speak like that very rarely.

Most fanfiction is written by American teenage girls. If you're writing about American characters, never mind, but if you're writing for Harry Potter, the characters are British. 'If you will' is a grating Americanism; and no one from any other country EVER, UNDER ANY CIRCUMSTANCES, says it. Even if they're imitating Americans. Another example is 'ass'; the Commonwealth equivalent is 'arse'. 'Ass' only refers to donkeys in the Commonwealth. The other common Americanism is 'damn'. 'Damn' on its own does not occur in English, except when shouted. The rest of the English-speaking world is much more colourful; in Australia the appropriate words are 'shit', 'bugger', and our favourite f-word. In England, 'blast' and 'bloody hell' are also used.

Spelling is extremely important. It's also incredibly easy to fix. On most word processing programmes, pressing F7 opens the spell checker. If your word processor doesn't have a spell checker, download OpenOffice. It's entirely free and works on virtuall every operating system. If a reader has to spend even a second trying to work out what a word is, you've lost them.

In Harry Potter, most incantations are the Latin for the desired result; the only two exceptions I can think of are Avada Kedavra, which believe is Arabic, and Wingardium Leviosa, which is nonsense. When writing your own incantations, do not, under any circumstances, add '-us' or '-um' to the end of an English phrase such as 'turnintoafroggius'. You will only look a fool. I am in my sixth year of studying Latin and am more than happy to translate for you.