In response to Rhiannon's post, I thought I'd try to come up with the ten rules I follow (or should follow) in my own writing. Here it goes.
1.Be true to your characters. No matter what it says in your outline, if two of your characters feel like getting into a relationship, a fight or a submarine, there's nothing you can do to stop it, so don't bother trying.
2.Picture every scene like you're actually watching it take place. Don't describe everything, but know everything, like where your characters are standing, what they're wearing, how much money they have on them - everything. (I don't always follow this one, even though I should)
3. RESEARCH. It can always be put off, but before you start working on getting something published, make sure there are no gross factual errors, because if there are, it will lead to: a) never being taken seriously or b) Being published and taken seriously until someone feels like bringing up the fact that twenty years ago, you misrepresented some cultural ceremony and then having everyone suddenly mad at you.
4. Take breaks. Not when you're on a roll, in the zone, or whatever you call it. In that state, even peeing is not necessary, unless you can take a notebook to the bathroom with you... but when you get to that stage where you're convinced that if you just stare at a blinking cursor for a little bit longer, you'll have an epiphany and be able to finish the whole book, go outside and get some fresh air. Have a drink. Relax. Take a notebook in case of epiphany, but don't even look at it unless you have something to write down.
5. Do not intentionally use big, convoluted words that nobody knows the meanings of. It doesn't make you look smart.
6. Learn how to correctly use punctuation. Don't try to use punctuation you don't understand.
7. Use your language correctly. I'm not saying that you should cut out words like 'ain't', if they fit with your story, but know the difference between your and you're, know your theres and your tos and be conscious that a scruple is not a unit of measure and pronounciated is not a word.
8. Pay attention to details and be consistent. If you're writing a letter from the perspective of somebody in the U.K., for example, remember that U.K. English, U.S. English and Canadian English are all slightly different. Make sure you're using the correct language for the situation. This likewise applies to clothing styles (don't go throwing capes around regency England unless you're sure that's what they were wearing), music (in 1999, no one was listening to 1999 - at least not on purpose) and just about anything else you can think of.
9. Give your characters flaws. No one wants to read about some hot guy with great teeth and perfect muscles who never does anything wrong and then gets the girl (unless the girl is a less than perfect brunette who's actually stunning in her own way and mistrusts men - and it says Harlequin on the cover).
10. Creep people out. Just a little.