If you are writing a series of any type, or a single book that has a highly complex setting or timeline, it is imperative that you keep track of all the details so that you can maintain consistency. Readers become outraged if characters do things they "shouldn't be able to". Discrepancies in descriptions (people or places) or dates drive some readers insane.
Every author figures out what method works best for them. Do you keep lists, draw charts? Do you enter cross-reference notes in electronic files of your stories? As long as you keep track of things, how you do it should be tailored to your working method--but be sure you do it!
Editors and copy editors catch some amusing or horrifying discrepancies. A continuing character whose name is spelled differently in different books. A woman whose pregnancy lasts over a year (yes, she was a normal human--the author had altered the time of the second book without taking into account how it would affect this secondary character who had become pregnant in the first). People whose eye color or other physical attributes change from book to book.
Here's a sample of a very basic World/Character Worksheet. You could start with this and add to it.
BOOK TITLE, Series name and number
Setting (Describe the "world" as a whole. Record a description of each place where action occurs.)
Timeline (All historical or current events mentioned in the story. Dates of events relevant to characters.)
Language/Special Terms (Any "alien" or made-up words. Also expletives and common exclamations. For example, do you write "Oh god" or "Oh God"?)
Naming Conventions (Are there "rules" for how people or places or things are named?)
~ Nicknames, forms of address
~ Appearance, age
~ Relationships to other characters
~ Personal history facts (birthplace, education, jobs, residences)
~ Personality traits, phobias, hobbies, special skills
If your story is a paranormal or fantasy, futuristic or set in an alien world, then you have even more information to keep track of in each of the above categories.
Authors, what are your favorite tricks and tips for keeping your world consistent and coherent?