More than staying focused, the thing is if you lose interest in your work the second NaNo ends - either hitting 50,000 or hitting December 1st - the whole endeavor goes for naught.
It's not that you have to force yourself to follow through. You have to like the work enough to see the value of continuing with it. Even just one nugget of a plot idea, one line, one moment by your characters, is enough to justify finishing the rough draft.
Writing is both work AND fun. Find the fun part, and you'll get through the work in no time.