I'm not sure how most DM's prepare for an eminent campaign, but I feel my methods are touched by that infamous teenage procrastination. Rarely do I have more than a general inkling of what's going to transpire during a game session, and even then it usually occurs the day of. On multiple occasions I've gone with a story arc I thought of just moments before.
I know the ramifications and problems of my world and therefore the session to session specifics seem to come naturally. Along with solving the ever present problem of "the PC's bypassed my dungeon completely..."
One of the DM's I most admire literally sits down at the table with a d20 and is able to unfold an enthralling story, no notes or paper in sight. While I personally aspire to this style, I have also played under DM's who take copious notes, come to the game with photo-copies, and make meticulously beautiful maps. There is definitely something to be said of a DM who can physically hand you a piece of parchment bearing a royal summons or a tattered treasure map.
Whatever the case, it is nearly impossible to successfully make a detailed outline of the plot because dnd entails so many random factors. And often the best plot devices and characters evolve from the PC's actions.
In this case I think procrastination, or the inability to sit down and plan has actually helped me become a better DM. Lack of fact encourages heightened creativity, making for interesting advances. What is lost is solid technicalities, buys the element of surprise. For your players and for yourself, keeping the game fresh