Rule #1. Hetalia characters are not gay. (With the possible exception of some.)
Rule #2. Hetalia ships have to make sense. (Check your history.)
Rule #3. Always go back and check for plot-holes, spelling/grammatical mistakes, etc. (I say this because personally, when I see a spelling mistake, it seems to kill the flow of the story.)
Rule #4. Unless they're 2p!, fem!, or going through some personality-altering thing, don't change their personalities. (Also with the exception to rule #5.)
Rule #5. If your fanfic's setting is modern-day, base the characters' reactions/personalities/etc. off of events that have happened recently.
Rule #6. Get creative! Don't just copy scenes from Hetalia! (Seriously, I have read some fanfics like that. Nicht gut.)
Rule #7. Breaking the 4th wall is ok, just don't do it excessively.
Rule #8. It doesn't matter who you're writing about, don't make them op (over-powered). (Limit: England/Norway/Romania with their magic.)
Rule #9. When writing, don't make it obvious you hate or love one character. (Obviously, if you're in the story, then that's fine.)
Rule #10. When writing accents, if you don't know it don't do it.
That's all for now! Hope you learned something!