1.) Every post must have a purpose. Either to advance the plot or advance character development, or both. Sometimes there can be a third purpose, but usually it's one of those two.
2.) Every post must be exactly and only as long as it needs to be. You hate it when you ask how long a teacher wants an essay to be, and he responds "As long as it needs to be." Well, he's right. Get the point across. Make the plot point. Fill in a few details on the character by means of showing them. Make it long enough to do those things, but no longer.
3.) Be sure there is a sense of place. You don't have to write four rambly paragraphs about the location. One or two sentences will often suffice, so long as you provide the other roleplayers/reader with an idea of where this is, what the atmosphere or milieu of the place is, and the like. It can be as simple as the character's perception of a room/area/etc.
4.) Be sure there is a sense of person. With few exceptions, there is no need to spell out the exact details of a character's outfit and physical appearance, but carefully chosen details can serve you very well - i.e. ink stains, coffeestains, uncombed hair, STYLE of dress (i.e. "He was dressed in soft, muted colors", or "She wore a flowing white skirt"), manner of speech (i.e. mumbling, avoiding eye contact, staring, commanding sharply, etc.), and such, when dispersed throughout the post, can paint the picture of a person without spending unnecessary paragraphs on verbose descriptions.
5.) Avoid discussing a character's memories or dreams too much. Done in moderation, in small dosages, and without the ever-annoying 'telling', memories or dreams can further shape your character in a roleplay. I suggest that if you include snippets of a memory or dream, they be exactly that - snippets, no longer than two sentences, average length ones, at that. Further, that these not be used in every post. More like once, possible a few times, in the course of a roleplay, at PIVOTAL moments, not whenever you feel the desire to include them.
6.) Write what needs to be written. What happens here? What is your purpose in writing this post? Fulfill it. Portray what needs to be portrayed. Don't worry if it is too short or too long. Proofread for grammar and spelling (and inconsistencies), and post.
7.) Typically, roleplay posts are in the third person, past tense. Use of the future conditional is advised against. Use your discretion when departing from the standard point of view in roleplay posts, and when in doubt, ask the GM of the roleplay in question - or simply refer to the rules.
8.) The roleplay post is typically expected to contain the point of view of one character only - yours - and if you are playing multiple characters who are not in the same location or conversation, it is considered considerate to clearly separate the segments. Sometimes, there is a need to include the actions or dialogue of NPCs as well, and in some roleplays, dependent on the GM and the other players involved, light forms of puppeting are considered acceptable for longer posts.
9.) Good roleplay posts are good writing. While there are some differences in the ways in which one is permitted to portray actions, thoughts, and characters' points of view, a good roleplay post will also demonstrate a good handle on vocabulary, syntax, effective use of detail, and other elements of style.
See also:
The Purpose of a Roleplay Post by Saladin