My campaigns grow organically. I start with a small local problem that low level heroes can wrap their minds about and stick their swords through, then I grow it from there. For example, the campaign that is coming to close (Dreams of Glory) has been very successful. The PC's started out as travelers from all across Faerun who had come to Shadowdale to petition Storm Silverhand to train them to become Harpers. Of course they didn't get to see her that early in their careers, instead they discovered that some girls had been kidnapped from some locals and set about to uncover the mystery. This led them on a 4 year long campaign featuring the Black Network as the BBEG in which they liberated Daggerdale, foiled a Zhentrim plot to place puppet rules in many dalelands, aided Cormyr in its time of dire need. As a campaign finale, they are infiltrating Darkhold to rescue the very girls they set out to find four years ago when the campaign started.
All that grew out of one little adventure I found on the web four and a half years ago.
My next campaign will be Gamma World and I hope to start it off much the same way.
Note that the central theme of the Dreams of Glory campaign is very simple and has remained the same the entire time I've run it. Go find the girls Kidnapped from Shadowdale and bring them home. Everything they have accomplished has been a result of their attempts to find the girls and recover them. You don't need complex bad guys, or intricate and sophisticated plots to run a great campaign, in fact I've found that the more simple storylines tend to be the most satisfying for the players.