AJ's suggestion of checking the popular fanon thread and avoiding it all is a very very good one actually!
I'd definitely second that.
And this is a good general thread actually
- If you want to write an AU, carefully consider why you are changing the setting, and why you choose the setting you do. At the moment, so many people are writing high school and college AUs because that's what they know about. Even more frustratingly, most of them relocate their stories to the US for no reason other than that's what they know. That doesn't serve the story or the characters. Have a reason for any places where you stray away from canon, and make that reason something other than "it's easier".
- Keep your characters in character. What I've always done on the rare occasions where I write fanfic is sit with the novel and flick between all of the relevant sections about that character, so that I am sure that I'm not inventing things or contradicting things that Hugo said. It's really frustrating as a reader to read fics that butcher the characters. A lot of the common problems with this are covered in the fanon thread, but things like writing about Les Amis, but having Enjolras as the only one that is interested in politics is just going to be frustrating.
- Don't skimp on research. This links into both points above really. This is a very rich fandom with a lot of material to work from. Make use of that and it will make your story immeasurably better. I promise, sitting on wikipedia reading about stuff that you never knew existed is a very good thing!
- Seek help. Beta readers are always good, and particularly so if you are new to writing in a fandom. A lot of the common mistakes can be avoided that way, as well as cleaning up the near inevitable expression mistakes. Abaisse is a great resource, both from existing threads and for any discussions you might want to start to help you as you go.
- Read good fanfic. Ask for recommendations, go away and read them, figure out what you like about how they were written. There are some really great pieces of work out there.
- If your fic involves shipping, you don't have to engage in "pair the spare", i.e. you don't have to have every other character paired up with another one. Make sure whatever you're doing is in character, and set up relationships rather than just announcing them. Equally, you don't have to write the same pairings that everyone else is doing. I love to find fic that is a little more original in this respect.
- When you post fic, make sure it has a good summary. And never ever apologise in your summary!
- When you post fic, ask for constructive criticism. Lots of people will just be like "Omg this is the best story ever!", even if it's really really not. Asking for constructive criticism will help you improve far more than overexuberant and often baseless praise.
All based on personal opinion of course, but that's what I would look for as someone who reads fic.