Globally as far as I understand it?
No text walls.
Get to know a forum and their rules by browsing and asking questions in the right places. Make sure that you give some thought to the proper sub-forum.
Here it's a bit unfortunate that MOST of the traffic is in BDF and if you were going to post something that's "news" even the family forum would get more traffic than there...I digress.
Don't multi-post threads in different forums
unless... it's bad decorum but there are a few scenarios where I'd overlook it.
Don't post threads and then promptly forget to interact with them further. I'm guilty of this sometimes so I rarely post them.
Gravebumping is frowned upon. Check dates plzzz.
If you really feel like it's something that people need to see or it's relevant then start a new thread and link the old one perhaps?
There are reasons where an "interment bump" (where it's about to be in the grave) might be alright but typically no. It's not like it can't be done but for some reason "forum decorum" dictates that it should not be done. It's an interesting Misc topic I've been meaning to start so perhaps soon. I don't fully understand why it shouldn't be done btw, it just...eh.
Make sure you confirm what is meant before you start an argument. Discerning moods/meaning/tone within the text format can be a challenge and it's better to give the benefit of the doubt primarily...unless you are certain it's a troll (then report, don't feed).
No trolling plz. No flaming plz.
Be mindful of your audience. Plz, pls, idk, b/c, np, ty, etc. may not be understood by older users and even when you are trying to make a joke it may be lost in translation. I use plz/pluzzz/plez/pl0x only for humor "btw".
Did I mention text walls?
This forum has different site specific "form" but generally speaking I'd say these apply to most forums.