All right! This year will be my fifth (fifth? Has it really been that long?) Nanowrimo, and I'll be going for my fifth win.
First, I second everything everyone else has said so far. Choose what works for you and use it!
Second, use the forums, even if the only reason you log on is for your local forum. This is particularly good if you live in a Nano-heavy area. You can get together to bounce ideas off each other, to have mini-word wars, to celebrate all your milestones. If you don't have a Nano-heavy area (or even if you do!) don't underestimate the forums. Use them. Ask questions about little things that are impossible to Google, talk to others who are writing your genre, play to procratinate (I do this way too often!), take advantage of them.
Speaking of the forums, get a writing buddy or enemy. If encouragement motivates you, get a buddy. If insults motivate you, get an enemy. (I'm an insult fan myself.) Get several of them. You can motivate anyone, but have one buddy/enemy (or several if you so choose; I usually have three per year) in particular to motivate you to write. Keep track of your word counts. IM each other. E-mail each other. PM each other via Nano. Have private word wars.
If you're new to Nanowrimo and want some extra encouragement through what looks like a huge jungle, get a mentor. Many veteran Nanos (including me) adopt new Nanowrimo participants and encourage them through what looks like a daunting task (okay, it is, but now it's almost second nature to me).
Write something that interests you! If you don't like your idea, don't worry. You're not stuck with what you started with. Just start on your new idea. Stuck with something? Throw in some ninjas or pirates, one of them disguised as your new main character, another disguised as a Portkey. Plan or don't plan; it's up to you. I'm not a fan of planning, which probably explains the coherence of my four novels and of this comment.
Tell everyone you know about this. They may think you're insane, but some of them just might join you! Plus there's nothing worse than letting yourself down at the end of the month by telling them you didn't finish.
Internet! Use it in moderation, though. Last year I cut off all my instant messengers until I hit my 1667 words for the day and finished my homework (yes, I'm a college student). Then I logged on and did word wars. If the Internet is a weakness for you, cut it off.
Take a break from your normal writing atmosphere! Do you normally write on your computer? Try writing by hand one afternoon. Are you a longhand writer? Try typing for a day. Watch the difference in your style. If it doesn't help, you can always go back, but sometimes a change for an hour or so can bring a fresh perspective if you're stuck.
Reward yourself. The reward system is up to you (Every 1000 words? 5000 words? Reaching your daily word count?), but promise yourself something after you've written x words. I like to reward myself with hot chocolate after reaching my word count for the day, and if I haven't fallen behind by the weekend, I go out for hot chocolate and write some of my novel by hand.
Make your writing space the best it can be. Keep it however you're comfortable. Make it clear to anyone you live with that you must not be interrupted when you're writing. (If you're a teen living with parents, this may not go over as well, but do explain that it's important to you.) If you listen to music while you write, keep it nearby so you won't have to dig for your fight scene music when the villain appears. Tell your friends that you won't be as available in November as you normally are.
...And that's it for now! More as it comes.