Writing Like An Intermediate/Advanced Roleplayer

Planning
You'd be surprised just how much it helps to jot down what the main points of your post is going to be. You could layout the rough content of the paragraphs, or you could write the thoughts, actions and speech your character may use, whatever is helpful.

Use More Descriptive Language
While you must leave some things up to the reader's imagination, that's not to say that you should abandon them in a text filled only with dialogue and nouns, leaving them to think up complete descriptions of things themselves. It is supposed to be you telling the story, not them.

"Don't:
Eleanor looked at the dress laid before her as it rested upon her bed. She picked it up and looked at it for what seemed like hours. Her eyes flicked across it over and over, absorbing every detail.

'Well?' he finally asked, 'Do you not like it?'
'Oh no! I love it!' She exclaimed, waving her hand in dismissal of the suggestion.

Do:

Eleanor looked at the shimmering dress laid gently before her as it rested on her soft bed. Yet then she picked it up and looked at it for what seemed like hours, a look of surprise delight resting upon her face. Her eyes flicked across it over and over, absorbing every detail. It was a silky and delicate green, sparkling and shining in the light.

'Well?' he finally asked, a doubtful look upon his face, 'Do you not like it, is it the color?'
'Oh no! I love it!' She exclaimed excitedly, waving her hand in polite dismissal of the suggestion.

Make Use of Similes, Metaphors and Personification
A simile is like a comparison, often making use of the word 'like' or 'as'. E.g. 'The color is like that of a raspberry.' A metaphor is when one thing is said to be another in order to achieve a greater effect or emphasis. E.g. 'Her eyes are windows to her soul.' The eyes aren't really windows, but this does create some more interesting sentence than simply naming the color or shape of the eyes. Personification is when you apply (usually) human traits/abilities (ect.) to something that is not human, again to make things more interesting. E.g. 'The cold clawed at them with its icy nails.'

Don't:
The hot sun burnt their skin as they walked through the scorching desert.

Do:
The sun's gaze was harsh and unrelenting, beating upon their shoulders like a red-hot whip. The desert was a hell from which they would never escape.

Don't Reveal Everything At Once
In order to keep people engrossed in the story you lay before them, you should not hand everything to them on a silver platter. That is, if you have introduced a new scene, or a new person, don't just reel off everything about them. Instead, reveal bit by bit throughout the paragraph, page, or even post.

E.g.

Don't:
Eleanor entered the room, she wore a long, white dress that hung to the floor and her light hair lay gently upon her shoulders as she looked at him with blue eyes. She had been in the gardens, singing with the birds, such a beautiful voice she had. She was tired and needed to sleep, such a long day it had been.

Do:
A gentle knock there was upon the wooden door, yet no answer could he give before it creaked slowly upon its hinges. A pale hand reached around it and followed a vision of white entering gracefully as if borne in by angel's wings. Unhurriedly, she closed the door behind her as if she was somehow unwilling to shut out nature, unwilling to place a divide between them as the birds still sang their songs outside, pleading for her to sing to them just once more. Then her eyes turned to his, a knowing glance she cast upon him, looking without permission but unhindered into his soul. There were as circles of blue staring at him, focusing on his true thoughts, his deep desires. A smile stretched across her lips as she walked closer to him, her dress brushing lightly across the floor.

'Eleanor—'
'Sh, my love.' She soothed, her voice tender and calm like the first morning rays of the sunrise.

Try Phrasing Things In Different Ways
Often, a sentence sounds much more interesting when said in a different way. Hard to explain without examples so here goes. Sometimes one way will sound better than the other, so once you get used to changing the sentence around, you can decide for each case, which sounds better (Hint: a lot of the time, you are changing from active to passive sentences and vice versa.)

The arrow pierced his heart, mortally wounding him. → He was mortally wounded by the arrow that pierced his heart.
He ran to save her as fast as he could. → To save her, he ran as fast as he could.

Try to Avoid Unwanted Repetition
Some repetition is okay, when it is intentional and sounds good. 'The world hated him, she hated him, and most of all he hated himself.' However, when you look over your posts and notice that you have used the same adjective times when describing your settings, characters and thoughts, or even worse, if you have repeated similes and metaphors, then you know you need to revise a little.

It doesn't take too long to use a thesaurus or take a moment to think of a different word, but it does make a post look better by far. Obviously you can't avoid repeating some things, just try and keep it minimal!

Make Dialogue More Than Idle Chat and React To Actions
Really get into your character, think you are your character when you post. Pretend what the other person has posted, what they would have said to your character, they have said to you. If you were that character, how would you react? You react in some way to everything that is said and done to you, so why should you ignore dialogue and actions in your post?

So, try and get a reaction appropriate for your character, don't move out of character. One of the worst things you can do is act out of character and make your profile meaningless.

Two Main Reactions
React physically or mentally or both! Think of your character as an actual person, and as a person, they will be happy, angry ect. Sometimes they might lash out, or even plant a kiss. Either way, make sure you react, keep things interesting, and make the thread more like one flowing story as opposed to two or more people fighting for their own stories, and not reacting to one another.

Afterthoughts
Don't forget at the end of the post, your character's afterthoughts. From all the actions and reactions, how do they feel? Has their opinion changed about the other character(s)? What are they now thinking?