Arsenic, giving mans a fucking lecture XD
While, completely correct, that guy's lectures are awesome, at little more immediate, is ask yourself questions. Read through your post, and, ask yourself this. "If I read this post, what else would I want to know to make the story better?" and then answer those questions. Then do it again. Always seek to answer some question. What are they doing, what are they wearing, where are they going, you can elaborate into many things.
Or, you can ask yourself "What in this post, gives other players something interesting and meaningful to respond too" and then see if you coul meaningfully respond to that, and go from there.