This is to expand upon Espen's list of RPing tips and such, but I'm going to aim at a particular point of it. Making your posts longer via detail. Some people feel that short and sweet is better, but sometimes that short and sweet can leave a sour taste in your mouth when you read it. There isn't enough to get into the character's head and feel what they feel. A good long post is good because you can see what that character is thinking, and their emotion hits you head on.
So how to do it? Well first, let's look at this first post:
Fred walked down the street. He walked into a bar. He sat down and ordered a beer. He sighed. Life was so bad.
Sort of wants to make you headdesk over and over, yes? There is a lot that post needs, besides a hard smack. We don't know anything about this character other than, well, life is bad. But we only know this because he sighed, and the sentence saying "Life was so bad." We need a bit more than this. We need the emotion that tells us that his life is bad, perhaps even the reason behind his glum feelings. We also need some detail in his surroundings. A little more descriptions of the actions wouldn't hurt either.
Fred shuffled down the street, his hands buried in his pockets, his head turned downward. His eyes focused glumly on the cracked sidewalk in front of him, each stone sweeping beneath him in an endless wave. When he looked up, he found himself in front of the Drifter's Tavern. Its glowing green and yellow neon sign burned the words into his eyes, his eyes having to blink to clear away the momentary hazy blur.
He walked into the bar, his brown eyes roaming over the interior dazedly. If anyone could see him now, they would see a broken man, his head hung low, circles around his eyes, his hair a mess. Because he felt like an absolute mess. His feet carried him over to a stool, trudging before he dropped into the first seat he reached and swung around to face the bartender. "Get me a beer," he said, his voice a gravelly tone that barely reached the bartender's ears. He watched the man at work with his somber gaze, re-evaluating his choice. He didn't really want a beer, he wanted something stronger. Something strong enough to kill the pain. Something to stop him from feeling anything for a long time. He let out a heavy sigh that did little to ease the pain in his chest. Could life possibly get any worse than this? He truly didn't know.
And there, we turned three (or four) basic actions into two decent sized paragraphs, simply by employing a little description of what the man was doing while he walked, what his surroundings were as he walked, and a description of what he found when he walked. In the second paragraph, we began to get a little bit in his thoughts, while still being mildly descriptive about what was happening with him.
In short, there are several things we can do to fluff up even the most basic reaction post. Emotions, details, and descriptive actions are some of these.
When you write your character, your first concern should be getting into the way he thinks about things. We can already figure what kind of person he is and what kind of personality he has. But how about the emotions? What is his current mood? Is he happy? Sad? Depressed? Why is he depressed? Did he lose someone he cared about out? Does he have any fond memories of this someone that lingers on the edge of his mind? Or maybe he's just so overcome with joy that he's bouncing all over the place, doing things just slightly out of character for his usual self, like waving at strangers and doing a little dance. Not only does this affect his actions, it also affects his thoughts. Why is he happy? Is it because he's excited about something? What is he excited about? Is it something he got, and he's anticipating trying it out? Or is it an event he's attending and he's already going through in his head what he's going to do and how it's going to turn out?
When you start delving into the emotions of someone and writing it out, it's okay to go into a bit of an emotional ramble. Go ahead and leave thoughts strewn throughout his normal mundane actions that would otherwise be one or two sentences. Often the thoughts lead into other thoughts, which subtly depict the character's change in mood, and also shows us readers what his personality is like. Our character can be skipping down the road, and he's thinking about the contest he's been invited to, because of how well he danced in the club. Now he's excited, thinking about what moves he's going to use. And he goes through several in his mind, tossing out the ones he doesn't like, and stashing away those he does like. Then somewhere in there he can start wondering, what if he doesn't do well? And start getting just the tiniest bit nervous, and maybe even thinking just a little too much on what if he fails. Maybe this bit causes his current action to slow a bit. And then maybe this nervousness leads to his worrying that a certain special someone watching him will see him fail. And what will she think if she sees him fail? Will she think any less of him than she did before? Now our character can start thinking more about his special someone, and less about the contest itself. Maybe he starts thinking on memories of things they shared. Now his train of thought has jumped from being excited, to thinking of his love interest.
Although I didn't actually write an example on this, our character went from excitement on contest > nervousness of failing > thinking about his girl. This is where emotional rambling can lead to, and can dramatically increase the length of a post. More importantly, it also gets us readers into the mind of the character, and we can start anticipating what our character is anticipating, and feeling the possible rejection he is feeling.
Second, description. What are our surroundings? Try to go into as much detail as you can on this. Invoke all the senses; sight, smell, hearing, touch, and taste. What do you see? Is it a cloudy day, or is it a bright and sunny, with large shadows being cast over the streets? How about the room you are in? Is it large, small, cluttered, clean, grimy, dim, bright? Is there anything interesting about the surroundings, such as the peeling paper on the walls, or the large coffee stain on the carpet under the desk? For that matter, what's even in the room? Well if we got cluttered, is it cluttered with furniture or junk? Does the brightness of the room give you a cheery feeling, or does the wallpaper make it feel dark and gloomy?
Now how about smell? Is there a scent in the room that makes you literally turn up your nose in disgust? (Like a nearby trashbin?) Is there a smell of cinnamon rolls wafting through the open window towards you? Does it smell just slightly stuffy in the room, making it hard for you to breathe?
And what do you hear? The sound of rain as each drop hits the sound of the metal roof with a dull thud? Or the hollow splash as it hits a puddle? That whistling whooshing sound of wind blowing through the trees? The distant sound of an airplane moving over head, or the sound of cars moving by? Heck even the sound of people chattering not far off. There is always some sound that makes itself distinct, even the total silence of hearing nothing.
Touch...the feeling of something sticky as your hand brushes over the surface of the table, that just makes you feel like you want to wash your hand. The feeling of the cold breeze as it blows by your unprotected arms, the feeling of your clothes being moved behind you that creates a sort of drag. How does the temperature of the air feel? Is it cold and makes your insides numb? How bout too hot, making you feel like you're baking, with the faint approach of a headache?
Taste...sometimes less used, but still one does have to eat, and sometimes one can taste the distinctive taste of iron in their mouth if a lung has been punctured. Or even that nasty taste that coffee, or yogurt, or whatever it is your character doesn't like, that makes him feel like throwing up. Look at Ten, he's always tasting things!
Last, descriptive actions. It's not enough sometimes to say that he walked down the street. Maybe he walked briskly down the street, or strode, or maybe he's depressed and trudged. Vary up your synonyms as they fit the situation and the emotions, and try to describe your actions thoroughly. Put in more little actions, unimportant ones that define your character, such as sticking his hands in his pockets as he saunters along, the adjusting of a bowtie, the incessant waving of the hands as he talks. Even the turning of the head as you regard the way another person is looking at you is a minor action, without actually doing anything.
If after all that, you still feel like your post needs something to give it content, then try adding a funny situation that isn't really relevant to the story, but challenges your character's personality. Such as while walking to the bar, he bumps into a stranger, and they have a little argument about it, or he does something minorly embaressing that somebody notices and comments about, before reaching his final destination.