The tone of my novel is pretty casual. It may be that your novel has a more formal tone to which my advice wouldn't apply. If so, good for you. I'm currently reading Kay's Fionavar Tapestry series--which is excellent by the way--and it trots out words I'm not sure I've actually heard spoken. Somehow, though, the formality is never a barrier, and instead builds into the high fantasy world he creates.
However, for me, rigid, overly formal writing is a problem. It doesn't fit with my novel, but is something I'm prone to. Things are looking up thanks to my writing group's diligently pointing out any offending patches they see. With that help, I'm now starting to notice patterns that I tend toward when I'm getting too formal.
- Don't avoid contractions -- I fought this one for a long time. My group suggested I use contractions more often. Something in me (with the voice of some writing teacher along the way) told me, "You cannot use contractions in your writing. It is weak and poor form!" Needless to say, once I got over the mental hump, my writing flowed far smoother.
- Big words -- One of my favorite sayings if "Why use a big word when a diminutive one will suffice?" That just about sums it up right there. In more casual writing, the length and complexity of words I choose sometimes drags things down, makes them too stodgy and technical. Anytime a word is more than a couple syllables, I now seriously consider whether it's really what I'm trying to say. Sometimes, the big words get to stay, but only when 1) they're doing the exact job they're intended for and 2) there isn't a viable, shorter alternative. (For the record, I'm hoping to someday write where I can totally cut loose with whatever high-falutin vocabulary I want, but that day isn't today!)
- Don't always look for synonyms -- Trouble comes when I try to avoid saying something plainly. I don't want to just say "It was green," that seems too bland, so I end up with a monstrosity like "It possessed a greenish pallor." None of the words are the end of the world individually (well, except "greenish" maybe) but the drive for synonyms ended up in a very bad place. Now don't get me wrong, variety in structure and word usage is important, but getting rid of every "was", "said" and "went" in your novel isn't going to make it stronger. Sometimes, the thing just was green.
Word delta: New handwritten chapter