Rule number one: Spam
Do not post messages that have no constructive content at all. It is developing, informing or in any other way improving then go ahead and post it. But messages such as shortnings (lol) or smileys (xD) shall not be posted as the only content of a message in any other board than the board - 'Off Topic Discussion'. This goes for topics and polls as well, obviously.
Rule number two: Offensive Posts
Do not post messages that, in any way, harm or have a negative influence to a player/user of the forum/server. This includes posting things that are either racist or discriminating. Apart from getting you a punishment, this will only make you less popular amongst players of the server.
Rule number three: Advertising
You may not advertise other SA-MP forums/others nor post illegal download-methods etc.
Rule number four: Double Posting
Do not double-post. This means posting several posts in a row, in the same topic. Simply press the modify-botton up in the right cornor.
Rule number five: Big Signatures
Signature restrictions are in place using forum settings, however if you use signatures that a normal person would conceive as obtrusive, it will be removed.
Rule number six: Quotes
Whenever a person has posted a picture within his post and you want to quote it - make sure to remove the picture, or replace it with a direct link.
It only fills up the page and there's no reason to have 15 copies of one picture, is there?
Rule number seven: Posting in restricted boards
Some boards are only meant for specific groups of people, such as the unban appeals (only appealers and admins, and the admin complaints (Only witnesses, admins and topic starters). Posting in these groups without being allowed to will result in a direct punishment.
Rule number eight: Posting Offensive Material
No posts of gore, extensive violence or pornography are permitted on the entirety of this forum. This is to comply with Google's terms that come with the adverts that help pay for the forum.
Rule number nine: Avatars
Avatars may not be more than 1024kb (One megabyte) in size. You would be utterly amazed with how much you can fit into 1024kb, even a long animation can be compressed down, they are after all only small images.