Although some writers have problems writing too many words for their novel, other writers find when they finish their first draft, they have too few words. Here’s some ways too increase your word count without making your story boring.
Flesh out any areas where you’re telling about a scene and not showing it. Every writer can find at least a few places where they’re doing that.
Develop layers of subplots. Subplots will make your novel more interesting. One word of caution – make sure your subplots connect in some way with the main story. You don’t want a bunch of bunny trails.
Develop minor characters. Spend some time turning your one dimensional characters into interesting people. Here’s a link to help you create interesting characters.
Don’t allow talking heads. Talking heads are stories where two characters are talking to each other on an empty stage. There’s no description of where they are or what they’re doing. If you find this in your story, give your characters bodies and a place to stand while they’re delivering their dialogue.
Add description. Too much description can slow down a story, but adding descriptions, especially descriptions that affect the characters in some way, will make your story richer.
Add interior monologue. Get inside your point of view character’s head from time to time. Let the reader know what she’s thinking.
Blow something up. There’s nothing like blowing up a character’s plans and goals to increase the word count of a story and to make it more exciting. Here’s a link that tells more about this.
Word Sharpeners 
Great tips! I found that with my NaNo 2009 Novel, I’m way short of an 80K – 90K novel, even though I won NaNo. When I go back to edit it here probably this fall, I’ll be using a lot of what you’ve suggested to expand my story.
Though, I have thought about making it longer by writing a second novella!
Normally I have to decrease my wordcount when I finish my manuscript, but with my current WIP, I know I will have to add wordcount. This article gives me ideas of things I can do both as I revise chapter by chapter and after I’m done when I review the novel as a whole. And the one I like best (especially since I write suspense novels) is to blow something up.
I hope this helps.
Great list – esp. that last one! I’ve never had a problem with too few words. Right now I’m doing my best to hack 50,000 off my WIP…
Too few words is always my problem. I tend to do the talking head thing.
Some good thoughts there. I’ll have a look at some of the other links you provide for further details. I found it interesting writing my first novel. You have an idea but until you write it, you really don’t know how long it is going to be – will it even be novel length? I suppose with experience you’d get better at judging this before starting. With my novel, I managed to hit around 60,000. It exceeds NanoWriMo’s definition of a novel but it could probably do with being a bit longer.