Keeping notes for your story is important. A list of the characters that appear, and how they are related to the protagonist and antagonist will help keep you from getting confused as the story progresses. If you have a lot of minor characters, these notes are important. The information in these notes can contain a few important details, or you can write up a complete dossier for each one. Remember, keep the back story of the characters to a minimum in your story. If something doesn’t relate to the plot or why a character acts a certain way in a certain situation, don’t add it to the story.
Notes on locations, any special technology, or weapons will help to keep things factual. There are readers out there who would happily rip apart your work if you use the wrong terminology. Keep track of books you use for research, along with website you visit, and magazine articles.