(From my old Groovy gamers account)
Some times a one liner is good at building valid dialog, because everyone seems to monologue in PbP. But that is the nature of the beast. Just make sure that if you do throw off a one line it's topical.
Sometimes I like to include what my character is thinking behind what he is saying. Just like in real life, we often go through different drafts of what we say(both before and after)as we say it. The key to good IC posting is to make sure you describe in vivid detail what you are trying to convey. My character(just like me) can say "Shut up!" in hundreds of different ways, with hundreds of correspondingly different meanings. Try to incorporate a literary equivalent of facial expression, body language, and tone into your posts. Think about your "thought-to-speech" process and try and translate it into a readable style.
I can type: "'Shut Up!' he snapped impatiently."
Or I can type...
RJ had dealt with bullies before. And quite frankly he was sick of them. What gives them the right to talk to anyone so disrespectfully. Something within Rj snapped, and before he could contain it..."SHUT UP!"...The words came quick and furious from his lips. And the dangerous flash in his eye, the way his fists clenched at his sides, told anyone who cared to watch; it was time to keep silent.