When I first began writing fiction, I was taught by one of my brilliant teachers to write BIG. (Which actually means big in word count.) He told us to plug in the research, to write lengthy descriptions, and just let the words flow. So that's what I usually do.
When I finish, I step back and look at the novel as a whole. I consider what needs to stay and what can go. What this means is that when I'm finished writing I usually end up cutting between 3,000 - 10,000 words. Sometimes the trimming happens in chunks. Sometimes it happens word for word.
If you have a complete novel that needs a little trimming, here are a few tips:
-- Look at each chapter and scene to make sure it advances the story. In my last two novels I've cut whole chapters after I realized that they were nothing more than wonderful research. Yes, there was action, dialogue, etc. but I knew they had to go when I cut them and it didn't change the plot. (Of course, usually I include these chapters when I send my manuscript in *hoping* my editor will see their brilliance. But usually he too agrees with the cut.)
-- I do "finds" on passive verbs and rewrite those sentences to make them tighter. Here is a great link for information on passive vs. action verbs.
--I also reread each scene to make it as tight as possible. It helps to remember each scene has a beginning (which draws you in), middle (action/reaction), and end (hook to keep you reading).
Here is a good link with tips for writing scenes.
-- I take out "saids" if it's clear who's talking. (You won't believe how many words can be cut this way!)
-- Make sure my scenes don't start too soon or trail on too long.
-- Cut out any descriptions of emotion. If it's obvious from the text how the person feels, I don't need to say it. (The reader should be crying from the drama of the scene not the words that state tears.)
Okay, those are some quick tips, but ones I think can help!