Focus only on the parts that are relevant to the campaign and what effort your players will put into the campaign themselves.
For example, there's no point in writing a five page history if you know that your players are going to rebel in reading it. When I write out material, I know my players have a tolerance to about two pages and I don't go beyond that limit.
Secondly, the more local you are in, the more detail you should create. If you expand out, then write less and less. For example, if you are basing your campaign in a small town or large village, you should have a map, some noted locations, a few NPCs, and some hooks for adventuring. But when you discuss the kingdom or region that the PC's are in, that should be a paragraph about the most. There's no point in statting out the good King when the PC's aren't going to be visiting him anytime soon.
Thirdly, if you run a railroad style campaign, make sure that you keep the campaign on the rails. If you run a sandbox style campaign, start with a small sandbox and then gradually expand the boundaries as the PC's discover new locations. Think of the model used in the Grand Theft Auto games. Several locations in the beginning of the game aren't available until the player has completed certain missions or progressed the story along at some point.
Lastly, prepare ahead of the campaign in general. Before you start your campaign, you should have some maps, NPC's, adventure hooks, and locations ready to go; however, if you don't, then go to RPGNow or various fan sites and download free maps, NPC's, locations, etc. Put them together and come up with something that ties it to your campaign.