In a perfect world, the substance of your posts would be all that mattered, but in our world, we all use heuristics (mental shortcuts) to determine the value and credibility of a post (and the one doing the posting) that includes the style of the post, as well. The suggestions below are based on research in the area of perceived intelligence and persuasion.
1) Check your Spelling. Unless you are still working in the 1990's, a spell checker can be used virtually everywhere you type. Use it!
2) Learn the most common grammar mistakes and don't make them. If you are one of those people who excelled in English and know the rules well, good for you. If you are like the rest of us, bookmark this website and read it when you can: http://public.wsu.edu/~brians/errors/errors.html.
3) Avoid redundancies. You don't need to say "true fact." A fact is assumed true otherwise it is not a fact. For 199 more redundancies, see http://grammar.about.com/od/words/a/redundancies_3.htm.
4) Talk like a grown up. When you tell a story, don't preceded a quote with "and I was like," "and she was like," "and then he was like." Use more descriptive verbs such as "said," "thought," "screamed," or one of many other verbs in place of "like."
5) Watch your mouth. In place of a swear word, use a cogent argument or a more accurate and descriptive word.
6) Avoid absolutes. It never rains here. I always have bad luck. I dance all the time... Yes it does, no you don't, and no you don't. This kind of communication is left over from our childhood days.
7) Avoid exaggeration. No, there were not "like a million people" at my party... 90 is a more accurate figure. Like absolute statements, exaggerated claims are most commonly made by children (and politicians).
8) Use punctuation. Start sentences with a capital letter, end sentences with punctuation, and use spaces. Don't type in all caps (outside of stressing a word or phrase).
9) Proofread. If you have a computer that will read text for you, use it to proof your writing (MACs have this). This way you can listen for errors that your eyes did not pick up.
10) Go easy with the texting shortcuts. Few people will judge you for throwing in an "LOL" or an "OMG," but when your posts read like license plates, people might question your ability to form a coherent sentence.