Plan your post. Have something in mind you want to post. Many of the posts on the Forums are for buying, selling, and trading items.
Find the right forum. There are forums for trading, selling, Clans Recruitment, Off Topic, Tech Support, and Forum Games where you may not make threads for In-Game transactions. Posting in the correct area will ensure the people you want to have read your post, will actually see it.
Title. The title of your post is the first, and sometimes only, part of your post that people will look at. Make sure it is accurate and specific. Don't post all in caps, it's annoying and demanding sounding. It is okay to sometimes put one word in caps so it stands out and attracts attention though.
Content. Always keep the content direct, on topic and specific. For example, if you are selling an item, stating that you are selling it, for what price, and whether the price is firm or negotiable. Also include any perks, such as rounding up, or whether you will go to a location near them to trade. Include how you wish to be contacted. Spell check your typing before posting, and use proper English with no abbreviations. Professional sounding posts will get more responses.