Please note, I will be adding to it over time; this is a work in progress.
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I wrote this guide assuming that you are a newcomer to the forums and would appreciate some advice on using/posting on the forum. Below is some advice as to what to do.

1. Your avatar
Arguably this is the very first thing that you should do when you come on to the forums. After you log on, you will see a white link saying 'Forum Preferences' in the top-right corner, just to the left of the search bar. Can't find that? It's on the right below the big blue bar at the top. (Thanks, Sket.) Click on it, pick an avatar, and remember to click 'Save'!

2. Posting on the forums

2.1. General rules
Take a look at the forum rules before you start.
Try to make your writing understandable; people who can't read it won't like it. Good grammar/speech helps people like you.
Keep it clean! Eurydice polices the forums and may punish you for swearing. (You're doing a great job, Eury.)
Keep it to the point. People don't want to see a great wall of text, because they will not bother to read it. If you must write a long post, put in a TL;DR (Too Long; Didn't Read) message at the bottom of the post.
If you can, go back and proofread. If you can't be bothered, that's not too bad.
Please don't directly insult people and start flamewars. Be polite. That just makes you look bad for being childish. If you really hate that person, keep quiet about it!
DO NOT RAGE! If you want to complain about something (AA/CE (rent)/heart pendants), don't do it here.

2.2. Your First Comment
By now, you will probably be wondering why on earth you even logged on. Find a subsection, find a topic, post something relevant. As a general rule of thumb, it's best to be honest and to answer properly.
Make sure you read through all the posts before yours! You may find an answer to your question/find someone else has already said that, and not need to post, or might find something else that is more important. (Thanks, Ubernerd)

2.3. Your First Topic.
Before you post anything new on the forums, CHECK THE SEARCH BAR! Often, you will find something relevant there, and will not have to post.
Make sure your topic goes in the correct forum subsection. Don't put a question about which Brandish line to use in the Treasure Vault.
If it is a suggestion, think it through properly beforehand.
Please do not post about:
Pets
Dual Wielding
CE price
Lockdown (thanks, Aget)
Removing Mist etc.

2.3.1 The search bar (thanks, Ubernerd)
Several important things regarding the search bar:
1. Do not 'necro' threads, ie. post on threads that went out of use ages ago, for example, anything on the third page of the forum or before (except the Bazaar), or anything with a date more than a month ago. Mind, not all necroing is bad, but make sure you have a very good reason to necro, or people will rage at you (Thanks, Fehzor).
2. Please, PLEASE read through the searched threads properly, so that you can find out as much as possible regarding the topic.

3. Making a good post
In order for people to want to read your post, you'll need to make it good-looking. Here are a few tips:
Try to not create a wall of text. Separate out sections of your post.
Make headings stand out. For example, try using HTML tags (click on 'More information about formatting options' at the bottom of the page) to make them different. Try using bold ( The word 'strong' in triangular brackets) to start with.
Keep the meaning clear, again. People don't want to read through your post more than once.

Now, go out and post already!

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Edit log:
#1 (23/9/12) Added parts regarding the search bar/necros and updated the list of topics not to post about, courtesy of Ubernerd and Aget respectively.
#2 (26/9/12) Added parts regarding 'good' necroing (Fehzor) and reading the posts properly (Ubernerd).
#3 (19/10/12) Added a more precise location regarding the search bar, courtesy of Sketman.