This got posted on a writing group board recently. Great advice. It's aimed at non-fic writers, but most of it applies to fiction as well.
Here are some fiction specific tips that I wished I'd known when I started writing novels.
1. Write the first draft without caring too much about voice, pacing, dialogue tags, etc. That can all be fixed in editing and rewrites (which you should be doing. Nobody writes a perfect first draft no matter what they may say.) Focus on your story, your character, the soul of your writing. This is where NaNoWriMo can help. Learn to just let go and let the story flow.
2. Never work on your original document. Always work on a copy. That way if you totally screw it up, you still have the original to help with the fixing. And remember to keep backups - online, on a portable hard drive, emailed to yourself, don't keep everything in one place.
3. Specific editing advice, for later, when you want to polish your writing:
Watch out for repetitive words. Repeating the same word too often makes your writing sound clunky.
Watch out for passive voice. "I was running across the grass" vs. "I ran across the grass." Anytime you use a "be" verb, stop and think if it's necessary. If you eliminate them, your action will feel more immediate, more personal.
Watch out for distancing the reader from your action. When you use words like "feel" "heard" etc, you are putting distance between the reader and the action. "I felt his hands clamp around my neck" vs. "His hands clamped around my neck."
Watch out for your particular verbal stutters. My first editor pointed out that I used "up" all over my manuscript. Most of them could be deleted without losing anything. My latest manuscript? I use directional words and "as well" way too often. Check for your own personal verbal stutters and eliminate as much as you can. Most of them aren't necessary.
Don't be afraid of chopping. Ken Rand's book, The 10% Solution, is a great resource. Cut 10% of your manuscript. Then go back and cut 10% more. You may think, I can't cut that much! But you'll be amazed how much you can cut without losing anything.
Don't be afraid of emotion. This one is hard for me. I shy away from revealing too much about myself through my writing. But when I let my guard down and let the emotions bleed in every word, my writing becomes more powerful. Yes, it can be raw writing that way. But it is also liberating.
Don't get hung up on the word count. Most publishers are flexible, within limits. Sometimes you do have to chop, chop, and chop some more. Sometimes you need to find places to expand your story. But whatever you do, make it meaningful, make it relevant, or it's just more words on a page.