The top of each submission form needs a red "Warning, you are not logged in! The submitter will be marked as anonymous, so either log in or don't forget to add author information!"

Alternatively, anonymous submissions could be required to have author information added, but the first solution would be better, because it would be clear before pressing "submit" and I'm sure many browsers will forget what you filled in if you log in while filling the form.

This is needed, because I have already twice had to change the author from anonymous to some username because they forgot they were not logged in. I also nearly made that mistake a few times.