As a series writer, I am sometimes asked how I keep everything straight. You know, how do you make sure Cousin Effie’s eyes aren’t brown in Book 1 and blue in Book 5?
The best way I have found to stay organized is to use a program called Scrivener. This allows you to keep all of your research (including websites), character notes, character designs, etc. all in the same location as your manuscript.
I also use what writers call a Series Bible, which is where you simply create one massive volume with every single tiny detail you have ever put into your novels. You can even hire people to create these for you if you have the money. This way, you never have to worry about remembering what your lead character’s third cousin twice removed was named–it will be in the Bible.