Mood, Feelings, and Emotions

Personally, one of my biggest suggestions is not to write when you're sad or mad. Sometimes, it really messes with the atmosphere you're trying to create within a book/poem/chapter. However, if you must write, I suggest using your current mood to your advantage.
For instance, if you're mad, put your character in your shoes, make them mad, and write. If it fits later on in the book, great. If it doesn't, no big deal. At least you got to write something. If you're sad, make the mood in the book somber. Relate to your characters, make them relate to you, and write. If you're all feeling the same emotions, the emotions come off as being extremely strong to the reader. If you're mad when your characters are mad, sad when they're sad, happy when they're happy, or thrilled when they're thrilled, the mood of the book goes from being only imaginary to being way more realistic.
When you're actually upset, you don't have to imagine what your character would do when they're upset. You're upset, and that automatically gives you an inside look at how they might, or definitely would, react. This goes for every single emotion, not just the aggravating ones. Believe it or not, it really does help set the mood in a very, very realistic fashion.