Part I - General Communication Pointers
Grammar and Spelling
Using correct grammar, typing out words in full and checking the spelling of your writing has a number of benefits when forum posting.
Firstly, it simply makes your communication easier to read. Languages have rules because there is a consensus on how certain structures affect meaning. Your reader wants to read effortlessly, not stop and try to puzzle out what you're saying. Having to do so causes the reader to experience a moment of frustration which makes them not enjoy reading your post.
Secondly, in many minds, whether consciously or not, poor grammar and spelling creates the impression that the writer is stupid, which can lead a reader to decide that the message is unimportant and not worth the time it takes to read.
Thirdly, poor grammar can obscure or even change the meaning of a statement. Consider "The panda eats shoots and leaves." This is a rather famous example and has a book named after it. One misused comma and we have "The panda eats, shoots and leaves." The meaning has changed.
This does not mean you have to use exquisitely formal grammar. Suit your tone to your post. Just don't use wrong grammar if you can avoid it.
If you know you are a poor speller, it can help to type your posts in Word or Open Office first. A spellcheck will catch most mistakes, at least the silly-looking ones. Homonyms (words that are pronounced the same but spelled differently) are something you will have to check yourself to make sure you've used the right form in the right context.
Grammar checkers can also be helpful; they won't produce a beautifully-written piece if you use auto-correction, but they will guide you to areas where your sentence structure might be possible to improve.
Typing posts in a local document before posting also means you won't lose your post when the forums kick you off, and you'll have an easily searchable record of your posts to refer to later, should you choose to save them.
Verbosity
TL;DR is an acronym I hate (it stands for "Too Long; Didn't Read"); it shows a contempt for the writer who has made an effort to communicate. But you want a reader to read your post and on the web, in particular, the reader as a rule has a lower tolerance for long posts because back-lit screens cause eyestrain and headaches and they have a game to go play.
A very famous writer once said "If you can cut a word out, cut it out." This is especially good advice for writing on the internet. As an exercise, imagine you're Batman in the 1970s TV series. You're being in peril and have to explain it to Robin while speaking in gasped, single-word bursts. "Boat... sinking... pull.... lever... stop... Penguin!" Those are keywords - a string of nouns and verbs that tell Robin exactly what he needs to know. If you can strip a sentence down to just the keywords, then add the minimum of words necessary to make it a proper sentence, it is a good web sentence.
But what if your character is verbose (or conversely, extremely terse)? Find a compromise that suits you, but remember that your post means nothing, no matter how perfectly in character it was, if no-one reads it and remembers the message.
White Space
Ever seen a post that is just a wall of white letters running from one side of the screen to the other all the way down the page? "Wall-of-text" is a phenomenon that makes reading feel intimidating. Wading through the material will be work and a reader doesn't want to work; they want to read and relax. "White space" is important in any long post for avoiding this.
"White space" is basically paragraph breaks. Leave a line between paragraphs so your writing doesn't look so dense.
Headings
If a post is very long, break it up with headings and subheadings. Decide on a format convention first - for example, main headings could be bold and underlined, and sub-headings could be just underlined - and be consistent with it.
Headings not only break up the page, but guide the reader to sections of interest and help reinforce the message in their memory.