Have you ever read a novel where there are description or continuity mistakes? I have read many novels where the spelling of the characters name changes or the hair colour changes. Yes these mistakes should be picked up in the editing process but sometimes a few do sneak their way through. Errors even happen in the novels of Stephen King
When writing fiction you need to keep track of your character description and development,plot, any subplots, time line, and to make sure any clues or important events are placed exactly where necessary.
To keep myself on track I like to use worksheets. Not every writer likes to use them but I find that this method works for me.
Here are a few examples that I hope you may find helpful
The Novel Notebook
Blank Novel Diagram
Novel Diagram Filled In As Example
Scene On Call Worksheet
Ten Point Plot Template
These are also interesting and helpful
Left Behind and Loving It Virtual Workshops 2007
Left Behind and Loving It Virtual Workshop 2008
Left Behind and Loving It Virtual Workshop 2009
If you scroll down I have another list of useful Worksheets below Writing Tools.