There are so many websites which offer resources to help you become a better writer. Use them often!
Here are a few important ones:
The Harry Potter Lexicon
Know your HP facts and histories. Jo Rowling herself uses the Lexicon for quick access to accurate information.
Merriam-Webster Online
Online dictionary and thesaurus
Harry Potter and the Mary Sue Litmus Test
What is a Mary Sue and how can you tell if your Original Character is one? Take the HP specific test by Priscilla Spencer, based on the
Original Mary Sue Litmus Test by Melissa "Merlin Missy" Wilson.
If you have found a site you've found useful, post a link in this thread. Please don't discuss these resources here. This is just for providing links.