I've seen some really cool formats, but I've no idea where to start looking things up for it (aside from the bottom cheat sheet that doesn't quite show ALL that BBCode is capable of). There are other sites that show some stuff, but I think it doesn't apply to this site (tried some code that didn't show the effects and only showed up as plain text).

In other words, how to prettify my posts, even just a little?

As far as I am aware, since this website was created from scratch not all bbcode works on it. If you have specific posts or character sheets you liked, it would be worth it to reach out to the person who posted it and just ask! A lot of people are friendly enough to share. You can also click the "Raw" link in the top right of posts to see what codes were used.

As for resources on the site, there are these two guides going over some things that aren't listed in the cheat sheet below posts, like tables (which you can see in some people's character sheets). Here and here. This section of the guild also has some formats and layouts other users have posted for general use.

@Yankee Thank you! This is really helpful.