There's a nifty little D&D article in the Legends & Lore column by Mike Mearls. Within, he talks about a "story bible". Basically, this is a written guide where the essentials are spelled out. The story bible can be handed out to writers, developers, or whomever so when they write an episode, chapter, adventure, campaign, or significant part of the story it doesn't break the overall continuity. For example, if we were talking about the Thundarr the Barbarian story bible, it would probably say that magic works alongside advanced technology, Moks hate water, the moon broken in half when a runaway comet went by, etc.
If you want to make your own story bible for an upcoming campaign, consider what sort of things should be included. Here's a guide to guide you in creating your guide...
- Setting - Where does this story take place? In the desert under seven suns? Within the ice caves beyond Thrakis? Beneath the emerald waves where the Frog-Men rule?
- Locations - Where are the adventures taking place specifically? The dark temple, inside a giant purple worm, the city-state of Koombash?
- Races - What's different about the races of your world? Are the Elves jealous of humanity for some reason? Do all the Dwarves have tattoos and vibrantly dyed mohawks? Will Lizardmen play a large role?
- Magic - Anything particularly noteworthy about spell-casting? You might want to create a different mechanical system for how magic works in your story.
- History - What happened ages ago? What happened last week? Were all the aristocracy beheaded? Did the Scarlet Magi cripple the assassin's guild until it worked exclusively for them?
- Culture - Do the inhabitants of Carcosa wear yellow masks? Why do the Hill Giants cover themselves in squid ink? Who prays to the Spider-God?
- Theme / Tone - What is this story about? Big picture. Is this going to be a campaign about knights in shining armor righting wrongs and saving damsels in distress or will this be a gritty, grimy campaign about the realm slowly sliding under a vampire's thrall?
- Concept art - Come up with a dozen images or so... artwork that speaks volumes about the story, who lives in it, what the mountains look like, is fire blue, and what your particular Halfling barbarians look like. Either find some images online, create some yourself, cut them out of magazines, scan them from books, or commission them from artists you know. The concept art can be seen by only you or the rest of your players (when the time is right).
Know these things from the start and they will help you stay consistent, inspired, and motivated to build upon your unique campaign.