No story is complete without characters. You may have a few or hundreds, or somewhere in between. But sometimes, as you write, you may forget a character name. Here are some ideas so to make it easier to remember or at least find if you can't remember.
1) First off, just make a list. It seems simple enough, but when you are writing, you may forget to do things like this. The inspiration comes and you just write. I like an excel sheet, myself, but a word processor sheet, such as Word or Google docs works just as well.
2) Add in where they appear in the book. Just by putting a page number where they appear is a great idea. At the very least, you might be able to find a description or how they are important to the story.
3) Relationships in the story. I don't mean just wife/husband relationship, but are they protagonist, a wizard, a mage, a warrior for some great nation, even how they are important to your main character. This would be important when you are dealing with interactions between characters if you are using a character very infrequently.
4)Descriptions. I like having these at a glance. Did they have blond hair or brown, blue eyes or green? If you just have to look in one place instead of flipping through drafts of your story, it makes that writing streak keep going.
Happy Writing!