How do you plan a whole campaign?

Points of reference? Chapter titles? Set up only, and impro later? Basing on the bio of the heroes? Do you have the main goal/conflict/villain at the very start? What's your prep vs impro balance? What's the key of your success?

Please answer only if you actually finished at least one long campaign.

EDIT:
Thanks for the answers, it fascinates me how different your approach can be.