1) Only write your character. Sometimes this isn't as easy as it sounds. Sometimes we want something to happen a certain way and we write other characters reactions without meaning to. This should be avoided. If you are playing Sam, then you should only write Sam's actions and reactions. If you come to a point where Dean has to react to what you've done, stop. Don't assume anything.
2) Don't contradict your partners. If your partner writes that something happens a certain way, don't write that it happens a different way. This only breeds confusion between the writers and readers alike, and hurts the story as a whole.
3) Be consistent. I know sometimes its hard, especially on long stories, to keep the details straight. But if you are in doubt, even a little, go back and make sure. Writing something, then a few chapters down, contradicting it, only hurts the story and breeds confusion.
4) Be realistic. Yes, I know, how realistic is two brothers fighting supernatural creatures to begin with? But despite that, the show is still set in the real world with real world consequences. For instance, if a bone is broken, it takes weeks to heal unless you give a plausable reason otherwise. Remember the old 50's Batman show where in any situation Batman just happened to have the exact gadget he needed (even named for the situation)? Yeah, things like that don't happen in the real world and makes it that much difficult to suspend disbelief when they are read. If you want something to happen, think of a reasonable, believable, way it could happen.
5) Sam and Dean are not contortionists. This kind of goes back to the Be Realistic tip, only it applies more to love scenes. Please, think about what positions you are actually putting the boys in while having sex. If you don't think you could do it, chances are, they couldn't do it either. If imagining yourself doing it makes you think 'ouch' yeah probably want to think of a different sex pose.
6) The story is about the plot, not the porn. I know, we all love writing Sam and Dean getting it on. But it is the acutal plot that makes or breaks a story. A story that is all porn and no plot is not only boring to write but also boring to read. There is no reason to rush into the sex. It will happen, trust me, and when it does it will be much better if the porn is part of the story, not the other way around.
7) Quality not quantity. Really, don't try to rush to write your responses as fast as possible, this is not a race. Take some time to think about it. You might be surprised what you can actually come up with when you take the time to, and it will help you grow as a writer.
8) Proper grammar and spelling is a MUST. You don't have to be the best writer in the world, but for the sake of your partners and readers, please use spellcheck and proper grammar. A few mispelled words here and there aren't really going to bother anyone. But constant spelling and grammar mistakes make things extremely hard to read, which not only makes it difficult for your partners but turns away readers as well.
9) Stay in contact with your writing partner. Whether through instant message, the board's chat room, or PM, it is a good idea to stay in contact with your writing partner and plan out a little how you'd both like the story to go. You don't have to plan out everything, and often surprises can add to the storyline, but at the same time they can also kill a story pretty fast. Also, if something happens in the story that you don't like, don't be afraid to tell your partner.
10) Show, don't tell. Ever hear the phrase a picture is worth a thousand words? Well, guess what, you can also create a picture through words. A few lines of dialogue and 'stage actions' might get the job done, but there is so much more you can do to bring life to the character and the scene. Here are a few examples of what you can include in a post to really give life to your character:
a) What is your character thinking? Yes, your character might be saying one thing, but he could be thinking something completely different. This can add a whole new layer to the story. What isn't being said is just as important as what is being said. It also helps your partner gain more insight into your character, and helps them better form their own replies. Though remember, unless mind reading is a skill your character possesses, use caution when reacting strongly to what another character is thinking.Thats all I can think of at the moment, more might be added at a later time. If anyone else has any helpful tips, please feel free to add them as well.
b)What is your character feeling? Again, very, very important to giving your character real personality. It is a good thing not to leave anything to chance when writing. It is so easy to misinterpret things, and misinterpretation only breeds confusion. We, as humans, see so much when we are looking at someone, reading their body language and facial expressions. When we're just looking at words on a screen, that is missing and needs to be replaced somehow. So telling your partner how your character is feeling, how they are reacting to the situation. It will give a lot more depth to your character and your story.
c) What is your character doing? This might seem simple, but its often not. Think about reading long lines of dialogue. What is the character doing during this dialogue? Are they standing there with a blank face and their hands at their sides? Most likely not. If your character is smiling, you should write that. If they are waving their arms around, or pacing back and forth, you should say that too. Chances are when you are writing you picture what they are doing in your head, so describe it. Don't leave it up to interpretation. Once again, leaving your characters actions to interpretation can lead to confusion and contradictions.