One- Have an idea! And don't wait until Halloween. Give yourself time to mull over the idea and see if you can get 50k words out of it. I have some favorite places to get ideas. The "Stuff You Missed In History Class" podcast, for one. They have lots of podcasts on random events in history they can really get your imagination going. That's where I got mine this year! Definitely think out of the box. Cast well known history characters in strange places! Women's suffrage in space; Prohibition in a dystopian future. The Writer's Digest is also a great place to find prompts. Listen to music, watch people in the mall, read the news. There's a million places to find an idea.
Two- Have at least a rough outline. How do you normally write? If you usually have a detailed outline to work from, make sure you write it out ahead of time. Even if you're a "pantser" like me or you aren't totally sure yet where the characters are going to take the story, at least jot out something. I would suggest at least four points: The beginning or setup, 2 middle points, and the end. I think the end is one of the most important, even if it's vague. Give yourself something to shoot for so your story doesn't wander off and run out of gas at 30k words!
If you can, finish your outline in the first couple weeks of October; again to give you time to mull it over or add points that come to you. If a scene pops into your head, outline it so you don't forget. November is going to be full of enough stuff without you having to recall stuff you thought of in October. (And in case you're wondering, yeah, my outline is done ...)
Along these same lines, if you have research to do, gather it ahead of time as well. You don't want to waste precious writing time scouring the Internet for information on Victorian bathrooms. (Let me save you some serious hunting- check out Victoriana.)
One of the best things about NaNo is just letting your writing go, but if you don't prepare a little, stress can get in the way of the fun. Get rid of potential road blocks now so you can really get out there and race!