I wanted to call this post: Educating your Writing Voice in Fiction by Managing your Mood. That however is appallingly verbose and not entirely appropriate. It is where this particular train of thought began though. In writing, particularly in fiction but also more generally, you have to choose the appropriate 'voice' for the text you are writing. Not the characters and speakers but for the text itself. Tension, calm, excitement and innumerable other senses and emotions need to be evoked as much by the style of writing as by the content of the words on the page. It's entirely true that you may be totally successful simply by employing a well thought out approach and some wise editing but there are a few things you can do to temporarily influence and educate your tone and style; you can inspire the appropriate mood in yourself.
Most of us are cognizant of the way our mood affects our speech. If you're flustered and stressed you make mistakes or are less concise; when you're angry you use more aggressive language and tend to be short and to the point, omitting simple pleasantries we usually employ without thinking. We do similar things when we write.
I started thinking more about this when listening to a song. I had some ideas going through my head and as the music approached it's crescendo those ideas become somewhat more solid, began to form into a coherent narrative and more importantly, the urgency and desperation in the music was reflected in the urgency and desperation of the text. Without really giving it any conscious thought i'd taken to using shorter sentences, making swift and to-the-point statements. It occurs to me that there are a huge number of things we use to affect our moods and if you know what type of piece you are writing in advance you can engineer your environment to promote the style of writing most appropriate.
If you need to be informative, take steps to relax, work in a well lit environment, play something classical in the background perhaps. If you're trying to express enthusiasm put on your favourite band, do something you're passionate about when you take a break. If you're writing about heartache then watch your favourite tear-jerker before you begin, remind yourself of the emotional content that (hopefully) you don't get to experience very often on a personal level.
This is no replacement for clever, detached editing, but it will make the process easier.