If you're brand new to a forum, lurk a while before beginning to post -- that way, you give yourself time to learn the rules, and the "flavor" of the forum -- less chance you'll mess up when you do
begin to post.
Don't double-post, or triple-post, or quadruple-post, etc. (Maybe the rosary threads are the exception, but even there -- give someone else a chance to jump in!)
Don't attach to your posts pictures that are so wide they force the post to occupy more than one screen's-width of space -- people then have to scroll back and forth to read the whole page of posts -- very annoying.
If you're quoting somebody else's very long post, only include in the quote the specific points you want to respond to, not the entire post. It is annoying to have to scroll past a whole series of posts where everyone has quoted the same incredibly-long post, and repeated all of it.
Don't hijack threads! Stay on topic. Especially don't try to turn the thread into a joke thread when the original thread-starter was asking a serious question.
Don't post the entire contents of really long articles. If an article is so long that you have to make six consecutive posts to fit it all, then it's too long -- provide a link, instead (and check whether the material you're posting is copyrighted -- maybe a link is all you want to post, anyway).