I meant to mention this in the old thread Ephyon linked, but I think the wiki can be used for this. We could write various articles in the shape of howto pages, such as Howto:Upload, Howto:Comment, Howto:Pixiv, Howto:Artist_entry etc.
Then we could link them whenever someone does something related to it (so if someone uploads something from Pixiv for the first time, a prompt could appear that says something along the lines of: "You just uploaded your first image from pixiv. Please take a moment to read the Howto:Pixiv!")
The great thing about using the wiki is that it already has a linking system in place and we can update it as new features or implicit/explicit rules are created. The only new feature would be a system that detects whether you've done certain actions and were shown the prompt. I'm not a programmer so I can't tell you how easy it would be to make, but it could be done with a "hide this prompt" thing, just like the current upload hint regarding deletions.