I have a few tips on keeping NaNoWriMo novels interesting and focused. It is tempting to just write extremely long dialogue to kill your word count, but it doesn't usually make for a good novel. (not that I know anything about that...)
Anyway, here are my tips...
Conflict. If there is no conflict, there is no story. Keep the conflict going in every scene. It can be internal conflict, dialogue conflict, a sword fight, anything, as long as there is conflict.
Character. Every scene ought to show something about some character. If the scene involves a character, and most scenes do, it should show some insight into their character. It can be as simple as their favorite color, or something deeper like a favorite childhood memory.
Quest. Keep your character's quest in mind. Not necessarily the principal plot, but make sure the characters are always pursuing something. Don't show scenes where they go shopping in the market or scavenge for food unless it has some impact on the plot.
Happy NaNoing everyone!